CHAPTER I.
Analogies between Animals and Vegetables.

AMONG the numberleſs objects with which the ſurface of this globe is covered and peopled, animals deſervedly hold the firſt rank, both on account of the relation they ſtand in to man, and of their acknowledged ſuperiority over vegetable and inanimated matter. The ſenſes, the figure, and the motions of animals, beſtow on them a more extenſive connection with ſur⯑rounding objects than is poſſeſſed by vegetables. The latter, however, from their expanſion, their growth, and the variety of parts which compoſe them, are more intimately related to external objects than minerals or ſtones, which are per⯑fectly inert, and deprived of every vital or active [2] principle. It is this number of relations alone which renders the animal ſuperior to the vege⯑table, and the vegetable to the mineral. Man, if we eſtimate him by his material part alone, is ſuperior to the brute creation only from the number of peculiar relations he enjoys by means of his hand and of his tongue; and, though all the operations of the Omnipotent are in them⯑ſelves equally perfect, the animated being, ac⯑cording to our mode of perception, is the moſt compleat; and man is the moſt finiſhed and perfect animal.

What a variety of ſprings, of powers, and of mechanical movements, are included in that ſmall portion of matter of which the body of an animal is compoſed! What a number of rela⯑tions, what harmony, what correſpondence a⯑mong the different parts! How many combina⯑tions, arrangements, cauſes, effects, and prin⯑ciples, all conſpiring to accompliſh the ſame de⯑ſign! Of theſe, we know nothing but by their reſults, which are ſo difficult to comprehend, that they only ceaſe to be miraculous from our habits of inattention and our want of reflection.

But, however admirable this work may ap⯑pear, the greateſt miracle is not exhibited in the individual. It is in the ſucceſſive renovation, and in the continued duration of the ſpecies, that Nature aſſumes an aſpect altogether incon⯑ceivable and aſtoniſhing. This faculty of repro⯑duction^*, [3] which is peculiar to animals and ve⯑getables; this ſpecies of unity which always ſubſiſts, and ſeems to be eternal; this generative power which is perpetually in action, muſt, with regard to us, continue to be a myſtery ſo pro⯑found, that we ſhall probably never reach the bottom of it.

Even inanimated bodies, the ſtones or the duſt under our feet, have ſome properties; their very exiſtence preſuppoſes a great number; and matter, the moſt imperfectly organized, poſſeſſes many relations with the other parts of the uni⯑verſe. We will not aſſert, with ſome philo⯑ſophers, that matter, under whatever form it appears, is conſcious of its exiſtence and of its relative powers. This queſtion belongs to me⯑taphyſics, of which we intend not here to treat. We ſhall only remark, that, being ignorant of the extent of our own connections with exter⯑nal objects, we cannot heſitate in pronouncing inanimated matter to be infinitely more ig⯑norant. Beſides, our ſenſations having not the moſt diſtant reſemblance to the cauſes which produce them, analogy obliges us to conclude, that dead matter is neither endowed with ſenti⯑ment, with ſenſation, nor with a conſciouſneſs of its own exiſtence. To attribute any of theſe [4] faculties to it, therefore, would be aſcribing to it the power of thinking, of acting, and of per⯑ceiving nearly in the ſame manner as we our⯑ſelves think, act, and perceive; which is equally repugnant to reaſon and to religion.

With inanimated matter, therefore, though formed of duſt and clay, we have no other relations than what ariſe from the general pro⯑perties of bodies, namely, extenſion, impene⯑trability, gravity, &c. But, as relations purely material make no internal impreſſion on us, and, as they exiſt entirely independent of us, they cannot be conſidered as any part of our being. Our exiſtence, therefore, is an effect of organi⯑zation, of life, of the ſoul. Matter, in this view, is not a principal, but an acceſſory. It is a fo⯑reign covering, united to us in a manner un⯑known; and its preſence is noxious. Thought is the conſtituent principle of our being, and is perhaps totally independent of matter.

We exiſt, then, without knowing how; and we think, without perceiving the reaſon of thought. But, whatever be the mode of our being, or of our thinking, whether our ſenſa⯑tions be real or apparent, their effects are not the leſs certain. The train of our ideas, though dif⯑ferent from the objects which occaſion them, gives riſe to genuine affections, and produces in us relations to external objects, which we may re⯑gard as real, becauſe they are uniform and in⯑variable. Thus, agreeable to the nature of our [5] being, it is impoſſible to doubt concerning the reality of thoſe diſtinctions and reſemblances which we perceive in the bodies that ſurround us. We may, therefore, conclude, without he⯑ſitation, that man holds the firſt rank in the or⯑der of nature; and that brute animals hold the ſecond, vegetables the third, and minerals the laſt. Though we are unable clearly to diſtin⯑guiſh between our animal and ſpiritual qualities; though the brute creation are endowed with the ſame ſenſes, the ſame principles of life and mo⯑tion, and perform many actions in a manner ſi⯑milar to thoſe of man; yet they have not the ſame extent of relation to external objects; and, conſequently, their reſemblance to us fails in numberleſs particulars. We differ ſtill more from vegetables; but we are more analogous to them than to minerals; for vegetables poſſeſs a ſpecies of animated organization; but minerals have nothing that approaches to regular organs.

Before we give the hiſtory of an animal, it is neceſſary to have an exact knowledge of the ge⯑neral order of his peculiar relations, and then to diſtinguiſh thoſe relations which he has in com⯑mon with vegetables and with minerals. An animal poſſeſſes nothing common to the mineral, but the general properties of matter: His na⯑ture and oeconomy, however, are perfectly dif⯑ferent. The mineral is inactive, inſenſible, ſub⯑ject to every impulſe, without organization, or the power of reproduction, a rude maſs, fitted only to be trode by the feet of men and of ani⯑mals. [6] Even the moſt precious metals, which derive their value only from the conventions of men, are regarded in no other light by the phi⯑loſopher. In the animal, the whole powers of nature are united. The principles with which he is animated are peculiar to him: He wills; he determines; he acts; he communicates, by his ſenſes, with the moſt diſtant objects; his body is a world in miniature, a central point to which every thing in the univerſe is connected. Theſe are his peculiar and invariable relations: The faculties of growth and developement, of reproduction and the multiplication of his ſpe⯑cies, he poſſeſſes in common with the vegetable kingdom.

Progreſſive motion appears to be the moſt diſtinguiſhing quality between an animal and a vegetable. We, indeed, know no vegetable that enjoys a progreſſive motion. But this mo⯑tion is denied to ſeveral ſpecies of animals, as oyſters^*, gall-inſects, &c. This diſtinction, there⯑fore, is neither general nor eſſential.

Senſation more eſſentially diſtinguiſhes ani⯑mals from vegetables. But ſenſation is a com⯑plex idea, and requires ſome explication; for, if ſenſation implied no more than motion conſe⯑quent upon a ſtroke or an impulſe, the ſenſitive plant enjoys this power. But, if by ſenſation we mean the faculty of perceiving and of com⯑paring ideas, it is uncertain whether brute ani⯑mals [7] are endowed with it. If it ſhould be al⯑lowed to dogs, elephants, &c. whoſe actions ſeem to proceed from motives ſimilar to thoſe by which men are actuated, it muſt be denied to many ſpecies of animals, particularly to thoſe that appear not to poſſeſs the faculty of progreſ⯑ſive motion. If the ſenſation of an oyſter, for example, differed only in degree from that of a dog, why do we not aſcribe the ſame ſenſation to vegetables, though in a degree ſtill inferior? This diſtinction, therefore, between the animal and vegetable, is neither ſufficiently general nor determined.

A third diſtinction has been derived from the manner of feeding. Animals have organs of apprehenſion by which they lay hold of their food; they ſearch for paſture, and have a choice in their aliment. But plants are under the ne⯑ceſſity of receiving ſuch nouriſhment as the ſoil affords them, without exerting any choice in the ſpecies of their food, or in the manner of ac⯑quiring it: The moiſture of the earth is the on⯑ly ſource of their nouriſhment. However, if we attend to the organization and action of the roots and leaves, we ſhall ſoon be convinced, that theſe are the external organs by which ve⯑getables are enabled to extract their food; that the roots turn aſide from a vein of bad earth, or from any obſtacle which they meet with, in ſearch of a better ſoil; and that they ſplit and ſeparate their ſibres in different directions, and [8] even change their form, in order to procure nouriſhment to the plant. A general diſtinction, therefore, between the animal and vegetable, cannot be founded on their manner of feeding.

From this inveſtigation we are led to conclude, that there is no abſolute and eſſential diſtinction between the animal and vegetable kingdoms; but that nature proceeds by imperceptible degrees from the moſt perfect to the moſt imperfect ani⯑mal, and from that to the vegetable: Hence the freſh water polypus may be regarded as the laſt of animals, and the firſt of plants.

After examining the diſtinctions, we ſhall now inquire into the reſemblances which take place betwee animals and vegetables. The power of reproduction is common to the two kingdoms, and is a reſemblance both univerſal and eſſential. This mutual faculty would induce us to think that animals and vegetables are beings of the ſame order.

A ſecond reſemblance may be derived from the expanſion of their parts, which is likewiſe a common property; for vegetables grow as well as animals; and, though ſome difference in the manner of expanſion may be remarked, it is nei⯑ther general nor eſſential; ſince the growth of ſome conſiderable parts of animals, as the bones, the hairs, the nails, the horns, &c. is the effect of a genuine vegetation; and the foetus, in its firſt formation, may be rather ſaid to vegetate than to live.

[9] A third reſemblance ariſes from the following fact: Some animals are propagated in the ſame manner, and by the ſame means, as vegetables. The multiplication of the vine-fretter, (puceron), which is effected without copulation, is ſimilar to that of plants by ſeed; and the multiplication of the polypus by cutting reſembles that of plants by ſlips.

We may now conclude, therefore, with more certainty, that animals and vegetables are beings of the ſame order, and that Nature paſſes from the one to the other by imperceptible degrees; ſince the properties in which they reſemble each other are univerſal and eſſential, while thoſe by which they are diſtinguiſhed are limited and par⯑ticular.

Let us next proceed to compare animals and vegetables in different points of view; for ex⯑ample, with regard to number, ſituation, mag⯑nitude, figure, &c. from which new inductions will ariſe.

Animals exceed plants in the number of ſpe⯑cies. In the claſs of inſects alone, there are, perhaps, a greater number of ſpecies, than of the whole ſpecies of plants on the face of the earth. Animals differ from each other much more than plants: It is the great ſimilarity of plants that has given riſe to the difficulty of diſtinguiſhing and arranging them, and to the variety of bo⯑tanical ſyſtems, which are much more numerous than thoſe of zoology.

[10] Beſide being more ſtrongly characteriſed, e⯑very ſpecies of animal is diſtinguiſhable from another by copulation. Thoſe may be regarded as of the ſame ſpecies which, by copulation, u⯑niformly produce and perpetuate beings every way ſimilar to their parents; and thoſe which, by the ſame means, either produce nothing, or diſſimilar beings, may be conſidered as of diffe⯑rent ſpecies. A fox, for example, will be of a different ſpecies from a dog, if nothing reſults from the intercourſe of a male and a female of theſe two animals; or, if the reſult be a diſſimilar creature, a kind of mule, as this mule cannot multiply, it will be a ſufficient demonſtration that the fox and dog are different ſpecies of ani⯑mals. In plants we have not the ſame advan⯑tage; for, though ſexes have been attributed to them, and generic diſtinctions have been found⯑ed on the parts of fructification; yet, as theſe characteriſtics are neither ſo certain nor ſo ap⯑parent as in animals; and, as the reproduction of plants can be accompliſhed by ſeveral methods which have no dependence on ſexes, or the parts of fructification, this opinion has not been ſuc⯑ceſsful; and it is only by the miſapplication of an analogy, that the ſexual ſyſtem has been pretended to be ſufficient to enable us to diſtin⯑guiſh the different ſpecies of the vegetable king⯑dom.

Though the ſpecies of animals be more nume⯑rous than thoſe of plants, the number of indi⯑viduals [11] in each ſpecies of the latter far exceed thoſe of the former. In animals, as well as in plants, the number of individuals is much great⯑er in the ſmall than in the large kinds. Flies are infinitely more numerous than elephants; and there are more herbs than trees. But, if we compare the quantity of individuals in each ſpe⯑cies, the number of the plant far exceeds that of the animal. Quadrupeds, for example, produce but few at a time, and at conſiderable intervals. Trees, on the contrary, produce annually an a⯑mazing quantity of ſeeds. It may be alledged, that, to render this compariſon exact, the quan⯑tity of ſeeds produced by a tree ſhould be com⯑pared with the quantity of germs contained in the ſemen of an animal; and then, perhaps, it would appear, that animals abound more in germs than vegetables. But, by collecting and ſowing the ſeeds of a ſingle elm tree, 100,000 young elms may be raiſed from the product of one year. Though a horſe, however, were fur⯑niſhed with all the mares he could cover in a year, the reſult between the production of the animal and of the plant would be very different. I avoid taking notice of the quantity of germs; becauſe of theſe, eſpecially in the animal, we have no certain knowledge, and becauſe the ſame ſeminal germs may exiſt in the vegetable; for the ſeed of a plant is not a germ, but a produc⯑tion as perfect as the foetus of an animal, and [12] which, like a foetus, requires only the expanſion of its parts.

To this may be oppoſed the prodigious mul⯑tiplication of ſome kinds of inſects, as the bee, a ſingle female of which will produce 30 or 40 thouſand. But, it ought to be remarked, that I am here ſpeaking in general of animals com⯑pared with vegetables. Beſides, the bee, which affords, perhaps, an example of the greateſt multiplication among animals, proves nothing a⯑gainſt the preſent doctrine; for, out of 30 or 40 thouſand flies produced by the mother-bee, there are but very few females, 1500 or 2000 males; and all the reſt are of neither ſex, and to⯑tally incapable of procreating.

It muſt be acknowledged, that ſome ſpecies of inſects, fiſhes, and ſhell-animals, appear to be extremely prolific. Oyſters, herrings, fleas, &c. are perhaps equally fertile as moſſes, and the moſt common plants. But, in general, moſt ſpecies of animals are leſs prolific than plants; and, upon comparing the multiplication of the different ſpecies of plants, we find not ſuch re⯑markable differences, with regard to number, as take place among animals. Some animals pro⯑duce great numbers, and others very few. But, in every ſpecies of plants, the quantity produced is always great.

From what we have already obſerved, it ap⯑pears, that, both in the animal and vegetable kingdoms, the ſmalleſt and moſt contemptible [13] ſpecies are the moſt prolific. In proportion as animals ſeem to be more perfect, the number of individuals decreaſes. Does the production of particular forms of body, neceſſary for the per⯑fecting of ſentiment, as thoſe of quadrupeds, and of birds, coſt nature more expence of organic particles than the production of inferior crea⯑tures?

Let us now compare animals and vegetables with regard to ſituation, ſize, and figure. Ve⯑getables can exiſt no where but on the earth. Moſt of them are attached to the ſoil by roots: Some, as truffles, are entirely covered with the ſoil; and a few grow under water. But the whole require a connection with the ſurface of the earth. Animals, on the contrary, are more generally diffuſed. Some inhabit the ſurface, and others the interior parts of the earth. Some ne⯑ver riſe above the bottom of the ocean, and o⯑thers ſwim in the waters. The air, the internal parts of plants, the bodies of men and of other animals, and even ſtones themſelves, are ſtored with inhabitants.

By the aſſiſtance of the microſcope, many new ſpecies of animals have been diſcovered. But, what is ſingular, we are not indebted to this in⯑ſtrument for above one or two ſpecies of plants. The ſmall moſs, of which mouldineſs conſiſts, is perhaps the only microſcopic plant which has been deſcribed. From this it would appear, that Nature has refuſed exiſtence to very ſmall plants, [14] while ſhe has created animalcules in the greateſt profuſion. But this opinion ſhould not be a⯑dopted without examination. Plants are ſo ſi⯑milar in their ſtructure, that it is much more difficult to diſtinguiſh them than animals. This mouldineſs, which we imagine to be only a very ſmall moſs, may be a foreſt or a garden conſiſt⯑ing of a multitude of different plants, though we are unable to diſtinguiſh them.

Animals and vegetables differ alſo with regard to ſize. There is a greater diſproportion between the bulk of a whale and that of one of theſe pre⯑tended microſcopic animals, than between the largeſt oak and the ſmall moſs mentioned above. Though bulk be only a relative attribute, it may be uſeful to know the limits within which na⯑ture has confined her productions. As to large⯑neſs, plants differ but little from animals. The quantity of matter in a whale and in a large tree is nearly equal; but, as to ſmallneſs, ſome men have pretended to have ſeen animals ſo extreme⯑ly minute, that a million of them collected in a heap, would not equal the ſmall moſs on a piece of mouldy bread.

The moſt general and moſt obvious diſtinc⯑tion between plants and animals ariſes from their figure. The form of animals, though infinitely various, has no reſemblance to that of plants: And, though the polypi, which, like plants, can be multiplied by cuttings, may be regarded as the link which connects the animal and vege⯑table [15] kingdoms, not only from the manner of their reproduction, but ſtill more from their fi⯑gure; yet there is no danger of miſtaking the one for the other. The operations of ſome ani⯑mals reſemble plants or flowers. But plants ne⯑ver produce any thing ſimilar to an animal; and thoſe wonderful inſects which make corals, would never have been miſtaken for flowers, if, by a fooliſh prejudice, coral had not been re⯑garded as a plant. Thus the errors we may commit in comparing plants and animals, are confined to a few objects which lie on the ex⯑tremities of the two kingdoms; and the farther we extend our obſervations, we ſhall be the more convinced, that the Creator has inſtituted no fixed limits between the animal and vegetable; that theſe two ſpecies of organized beings poſ⯑ſeſs a greater number of common properties than of real differences; that the production of an animal requires, perhaps, a ſmaller exertion of Nature than the production of a vegetable; or rather, that the production of organized bodies requires no immediate exertion at all; and, laſtly, that animation, or the principle of life, inſtead of a metaphyſical ſtep in the ſcale of be⯑ing, is a phyſical property common to all matter.

CHAPTER II.
Of Reproduction in general.

WE ſhall now more cloſely examine this property common to the animal and vegetable; this faculty of producing beings ſi⯑milar to themſelves; this ſucceſſive chain of in⯑dividuals which conſtitutes the real exiſtence of the ſpecies: And, without limiting our reſearch to the generation of man, or of any particular animal, let us contemplate the general phaeno⯑mena of reproduction; let us collect facts, and enumerate the various methods employed by Nature for the renovation and tranſmiſſion of organized exiſtences.

The firſt, and apparently the moſt ſimple me⯑thod, is to aſſemble in one body an infinite num⯑ber of ſimilar organic bodies, and to compoſe its ſubſtance in ſuch a manner, that every part ſhall contain a germ or embryo of the ſame ſpecies, and which might become a whole of the ſame kind with that of which it conſtitutes a part^*. [17] This apparatus appears, at firſt ſight, to ſuppoſe a profuſion of expence. Such magnificence, however, is not uncommon in Nature. It is diſ⯑cernible even in the more common and inferior ſpecies, as in worms, polypi, elms, willows, and many other plants and inſects, every part of which contains a whole, and, in order to become a plant or an inſect, requires only to be unfold⯑ed or expanded. Conſidering organized bodies under this point of view, an individual is a whole uniformly conſtructed in all parts, a col⯑lection of an infinite number of particles every way ſimilar, an aſſemblage of germs or minute individuals of the ſame ſpecies, which, in certain circumſtances, are capable of being expanded, and of becoming new beings like thoſe from which they were originally ſeparated.

This idea, when traced to the bottom, diſco⯑vers a relation between animals, vegetables, and minerals, which we would not have ſuſpected. Salts, and ſome other minerals, conſiſt of parts ſimilar to one another, and to the whole. A grain of ſea-ſalt, as we diſtinctly perceive by the microſcope, is a cube compoſed of an infinite number of ſmaller cubes^*, which, as we diſco⯑ver [18] by a larger magnifier, are themſelves com⯑poſed of ſtill ſmaller cubes. The primitive and conſtituent particles of this ſalt muſt, therefore, unqueſtionably conſiſt of cubes ſo minute, that they will for ever eſcape our obſervation. Plants and animals, which poſſeſs the power of multi⯑plying by all their parts, are organized bodies compoſed of ſimilar organic bodies, the primi⯑tive and conſtituent particles of which are alſo organic and ſimilar. Of theſe we diſcern the accumulated quantity; but we can only recog⯑niſe the conſtituent particles by reaſon and ana⯑logy.

From this view, we are led to conclude, that there exiſts in nature an infinity of organic, li⯑ving particles ^*, of the ſame ſubſtance with orga⯑nized beings. A ſimilar ſtructure we have al⯑ready remarked in more inanimated matter, which is compoſed of an infinite number of mi⯑nute particles that have an exact reſemblance to the whole body. And, as the accumulation perhaps of millions of cubes are neceſſary to the [19] formation of a ſingle grain of ſea-ſalt that is per⯑ceptible by our ſenſes, an equal number of ſimi⯑lar organic particles are requiſite to produce one of thoſe numberleſs germs contained in an elm, or in a polypus. A cube of ſea-ſalt muſt be diſ⯑ſolved before we can diſcover, by means of cryſtallization, the minute cubes of which it is compoſed: In the ſame manner, the parts of an elm or of a polypus muſt be ſeparated, before we can recogniſe, by means of vegetation, or ex⯑panſion, the ſmall elms or polypi contained in the different parts of theſe bodies.

The difficulty of aſſenting to this idea proceeds from the well known prejudice, that we can on⯑ly judge of the compound by the ſimple; that, to diſcover the organic ſtructure of any being, it muſt firſt be reduced to its ſimple and unorganic parts; and that hence it is more eaſy to conceive how a cube muſt neceſſarily be com⯑poſed of other cubes, than how a polypus can be compoſed of other polypi. But, if we ex⯑amine attentively what is meaned by ſimple and compound, we ſhall find, that in this, as in every thing elſe, the plan of Nature is very different from the groſſneſs and imperfection of our con⯑ceptions.

Our ſenſes, it is well known, convey not to us exact repreſentations of external objects. When we want to calculate, to judge, to com⯑pare, to weigh, to meaſure, &c. we are obliged to have recourſe to foreign aid, to rules, to prin⯑ciples, [20] to uſages, to inſtruments, &c. All theſe adminicles are efforts of human genius, and be⯑long more or leſs to the abſtraction of our ideas. This abſtraction, with regard to us, conſtitutes the ſimplicity of things; and the difficulty of re⯑ducing them to this abſtraction is the compound. Extenſion, for example, being a general and ab⯑ſtract property of matter, is not much com⯑pounded. In order, however, to judge concern⯑ing it, we have imagined ſome extenſions to have no thickneſs, others to have neither thick⯑neſs nor breadth, and points, which are exten⯑ſions without being extended. All theſe abſtrac⯑tions have been invented as ſupports to the un⯑derſtanding; and the few definitions employed in geometry have given riſe to numberleſs pre⯑judices and falſe conceptions. Whatever is re⯑ducible under any of theſe definitions is called ſimple; and ſuch things as cannot be eaſily re⯑duced to this ſtandard are conſidered as complex. Thus, a triangle, a ſquare, a circle, a cube, and all thoſe curves of which we know the geome⯑trical properties, are regarded as ſimple. But every thing which we cannot reduce under theſe ſigures, or abſtract rules, appears to us to be com⯑plex. We never reflect, that all theſe geometri⯑cal ſigures exiſt no where but in our own ima⯑ginations, or that, if they are ever found in Na⯑ture, it is only becauſe ſhe exhibits every poſ⯑ſible form; and the appearance of ſimple figures, as an exact cube, or an equilateral pyramid, is, [21] perhaps, more difficult and rare to be found in nature, than the complex forms of plants or of animals. It is in this manner that we perpetu⯑ally conſider the abſtract as ſimple, and the real as complex. But, in nature, no abſtract exiſts; nothing is ſimple; every object is compounded. We are unable to penetrate into the intimate ſtructure of bodies. We cannot, therefore, de⯑termine what objects are more or leſs complex, unleſs by the greater or leſs relation they have to ourſelves, and to the reſt of the univerſe. For this reaſon, we regard the animal as being more complex than the vegetable, and the vegetable than the mineral. With reſpect to us, this no⯑tion is juſt; but we know not whether the ani⯑mal, vegetable, or mineral, be, in reality, the moſt complex or the moſt ſimple; and we are igno⯑rant whether the production of a globe or a cube requires a greater effort of Nature than that of a germ, or an organic particle. If we were to indulge in conjectures upon this ſubject, we might imagine the moſt common and moſt nu⯑merous objects to be the moſt ſimple. But this would make animals more ſimple than plants or minerals, becauſe the former exceed the latter in number of ſpecies.

But, without dwelling longer on this ſubject, it is ſufficient to have ſhown, that all our no⯑tions concerning ſimple and compound are ab⯑ſtract ideas; that they cannot be applied to the complex operations of nature; that, when we [22] attempt to reduce all bodies into elements of a cubical, priſmatic, globular, or any other regular figure, we ſubſtitute our own imaginations in oppoſition to real exiſtences; and that the forms of the conſtituent particles of different bodies are abſolutely unknown to us; and, of courſe, we may believe or ſuppoſe that organized beings are compoſed of ſimilar organic particles, as well as that a cube conſiſts of other cubes. We have no other method of judging but by experience. We know that a cube of ſea-ſalt is compoſed of many leſſer cubes, and that an elm conſiſts of a great number of minute elms; becauſe, if we take a piece of a branch, of a root, of the wood ſeparated from the trunk, or a ſeed, from all theſe a new tree is produced. The polypus, and ſome other ſpecies of animals, may likewiſe be multiplied by cuttings ſeparated from any part of their bodies; and, as our rule of judging in both caſes is the ſame, why ſhould we form a different opinion concerning them?

The above reaſoning renders it extremely probable, that there really exiſts in nature an infinite number of ſmall organized beings, every way ſimilar to thoſe large organized bodies which make ſuch a conſpicuous figure in this world; that theſe ſmall organized beings are compoſed of living organic particles, which are common both to animals and vegetables, and are their primary and incorruptible elements; that an aſſemblage of theſe particles conſtitutes an [23] animal or a plant; and, conſequently, that re⯑production or generation is nothing but a change of form, effected ſolely by the addition of ſimi⯑lar particles; and the death, or reſolution of organized bodies, is only a ſeparation of the ſame particles. Of the truth of this doctrine, not a doubt will remain, after the proofs de⯑livered in the following chapters are peruſed. Beſides, if we reflect on the growth of trees, and conſider what an immenſe maſs is produced from ſo ſmall an origin, we muſt be perſuaded that this increaſe of matter is effected by the ſimple addi⯑tion of organic particles which are ſimilar to one another, and to the whole. The ſeed firſt pro⯑duces a ſmall tree, which it contains in minia⯑ture within its coats. At the top of this ſmall tree a bud is formed, which contains the tree that is to ſpring the next ſeaſon; and this bud is an organized body ſimilar to the ſmall tree of the preceding year. The ſmall tree of the ſe⯑cond year, in the ſame manner, produces a bud which contains a tree for the third year; and this proceſs uniformly goes on as long as the tree continues to vegetate: Buds are likewiſe formed at the extremity of each branch, which contain, in miniature, trees ſimilar to that of the firſt year. It is evident, therefore, that trees are compoſed of minute organized bodies ſimilar to themſelves, and that the whole individual is formed by a numerous aſſemblage of minute and ſimilar individuals.

[24] But, it may be demanded, were not all theſe minute, and ſimilarly organized bodies, contain⯑ed in the ſeed? and may not the order of their unfolding be traced from that ſource? for it is apparent, that the firſt bud was ſurmounted by a ſimilar bud, which was not expanded till the ſecond year, and the third bud was not unfold⯑ed till the third year; and, conſequently, the ſeed may be ſaid to have really contained the whole buds which would be formed for 100 years, or till the diſſolution of the plant: It is alſo appa⯑rent, that this ſeed contained not only all the ſmall organized bodies which muſt in time have conſtituted the individual tree itſelf, but like⯑wiſe all the ſeeds, and all the individuals which would ſucceſſively ariſe, till the final deſtruction of the ſpecies.

This, indeed, is a capital difficulty; and we ſhall therefore examine it with the greater at⯑tention. It is true, that the ſeed produced a ſmall tree the firſt year, ſolely by the unfolding of the bud or germ which it contained, and that this ſmall tree exiſted in miniature in the bud. But it is not equally certain that the bud of the ſecond year, and thoſe of the ſucceeding years, nor that all the ſmall organic bodies, and the ſeeds which muſt have been formed till the end of the world, or the deſtruction of the ſpecies, were contained in the firſt ſeed. This opinion ſuppoſes an infinite progreſſion, and makes every individual a ſource of eternal generations. The [25] firſt ſeed, for inſtance, muſt have included all the plants of its ſpecies which have exiſted, or ever will exiſt; and the firſt man muſt have contained in his loins all the men who have ap⯑peared, or will appear, on the face of the earth. Every ſeed, and every animal, according to this doctrine, muſt have included in its own body an infinite poſterity. If we yield to reaſonings of this kind, we muſt loſe ſight of truth in the labyringths of infinity; and, in place of ſolving, or of throwing light upon the queſtion, we will involve it in tenfold obſcurity. It is removing the object beyond the reach of our viſion, and then complaining that we cannot ſee it.

Let us inveſtigate the nature of the ideas of infinite progreſſion and expanſion. How do we acquire them? In what do they inſtruct us? We derive the idea of infinity from the idea of what is limited. It is in this manner we obtain the ideas of infinite ſucceſſion, and geometrical infi⯑nity: Every individual is a unit; ſeveral indivi⯑duals make a limited number; and a whole ſpe⯑cies is to us an infinite multitude. From the ſame data which we have demonſtrated the non⯑entity of geometrical infinity, we might prove, that infinite ſucceſſion, or propagation, reſts on no firmer baſis; that it is only an abſtract idea, a mere deduction from the idea of finite objects, by lopping off the limits which neceſſarily ter⯑minate every magnitude^*; and, of courſe, that [26] every opinion which infallibly leads to the idea of actual exiſtence, upon no better authority than what is derived from geometrical or nume⯑rical infinity, ought to be rejected.

The partizans of this opinion are now redu⯑ced to the neceſſity of acknowledging, that their infinity of ſucceſſion and of multiplica⯑tion is only an indeterminable or indefinite number. But, ſay they, the firſt ſeed, of an elm for example, which weighs not a grain, actually contains all the organic particles requi⯑ſite for the formation of this tree, and of all the individuals of the ſame ſpecies which ſhall ever appear. Is this a ſolution of the difficulty? Is it not cutting the knot, in place of untying it?

When, is reply to the queſtion, how beings are multiplied? it is anſwered, that the multi⯑plication was compleated in the creation of the firſt individuals, is not this both an acknow⯑ledgment of ignorance, and a renouncing of all deſire of farther improvement? We aſk how one being produces its like? and we receive for anſwer, that the whole was created at once. A ſtrange ſolution; for, whether only one or a thouſand generations had paſſed, the ſame diffi⯑culty remains, and, inſtead of removing it, the ſuppoſition of an indefinite number of germs, all exiſting and contained in a ſingle germ, increa⯑ſes and renders it altogether incomprehenſible.

I allow, that it is much eaſier to find fault, than to inveſtigate truth, and that the queſtion [27] concerning reproduction is perhaps of ſuch a ſubtile nature, as not to admit of a full and ſatis⯑factory explication. But we ought at leaſt to inquire whether it be altogether inſcrutable; and, in the courſe of this inquiry, we will diſcover all that can be known, and the reaſon why we can know no more.

Queſtions or inquiries are of two kinds; the firſt regard primary cauſes, the other particular effects. If, for example, it be aſked why mat⯑ter is impenetrable? we muſt either return no anſwer, or reply by ſaying, that matter is impenetrable, becauſe it is impenetrable. The ſame anſwer muſt be made, if we inquire into the cauſe of gravity, of extenſion, of the inertia of bodies, or of any general quality of matter. Such is the nature of all general and abſtract qualities, that, having no mode of com⯑paring them with other objects in which they do not exiſt, we are totally incapable of reaſon⯑ing concerning them; and therefore all inqui⯑ries of this kind, being beyond the powers of human intellect, are perfectly uſeleſſs.

But, on the other hand, if the reaſon of par⯑ticular effects be demanded, we are always in a condition to give a diſtinct anſwer, whenever we can ſhow that theſe effects are produced by one of the general cauſes; and the queſtion is equal⯑ly ſolved, whether the particular effect proceeds immediately from a general cauſe, or from a [28] chain of ſucceſſive effects, provided we have a clear conception of the dependence of theſe ef⯑fects upon each other, and of their mutual rela⯑tions.

But, when a particular effect appears not to have any dependence upon more general effects, or has no analogy to effects already known, we are then totally unable to give any explication of it; becauſe we have no ſimilar object with which it can be compared. We cannot explain a general cauſe, becauſe it equally exiſts in every object; and, on the contrary, we can give no account of a ſingular or iſolated effect; becauſe the ſame qualities exiſt not in any other ſubject. To explain a general cauſe, we muſt diſcover one ſtill more general; but a ſingular and detached effect may be illuſtrated by the diſcovery of an analogous effect, which experience or accident may exhibit.

There is ſtill another kind of queſtion, which may be called a queſtion of fact. For example, why do trees, dogs, &c. exiſt? All queſtions of this kind are perfectly inſoluble; for thoſe who ſolve them by final cauſes conſider not that they miſtake the effect for the cauſe: The relation of particular objects to ourſelves has no connection with their origin. Moral affinity or fitneſs can never become a phyſical reaſon.

Queſtions in which we employ the word Why, ought to be carefully diſtinguiſhed from thoſe [29] in which we employ How, and ſtill more from thoſe in which we ought to uſe the words how much or how many. Why always relates to the cauſe of the effect, or to the effect itſelf; how re⯑lates to the manner in which the effect happens; and how much relates to the meaſure or quantity of the effect.

Theſe diſtinctions being eſtabliſhed, let us now examine the queſtion concerning the reproduc⯑tion of beings. If it be demanded why animals and vegetables continue their ſpecies? we clear⯑ly perceive that this is a queſtion of fact, and therefore that it is uſeleſs and inſolvable. But, if it be aſked how animals and vegetables are re⯑produced? we are enabled to ſolve the queſtion, by giving the hiſtory of the generation of every ſpecies of animal, and of the reproduction of e⯑very ſpecies of plant: After tracing, however, every poſſible method of propagation, and ma⯑king the moſt exact obſervations, we have only learned the facts; but have not diſcovered the cauſes: And, as the means Nature employs in multiplying and continuing the ſpecies, ſeem to have no relation to the effects produced, we are ſtill under the neceſſity of aſking, by what ſecret cauſe ſhe enables beings to propagate their kinds?

This queſtion is very different from the firſt and ſecond. It admits of nice ſcrutiny, and e⯑ven allows us to employ the powers of imagina⯑tion. It is, therefore, by no means inſolvable; [30] for it belongs not to a general cauſe. Neither is it ſolely a queſtion of fact: And, if we can conceive a method of reproduction, depending on primary cauſes, or which, at leaſt, is not re⯑pugnant to them, we ought to be ſatisfied with it; and the more relation it has to the other ef⯑fects of Nature, it will reſt upon a firmer baſis.

By the nature of the queſtion, then, we are permitted to form hypotheſes, and to chuſe that which appears to have the greateſt analogy to the other phaenomena of nature. But we ought to reject every hypotheſis which ſuppoſes the thing to be already accompliſhed, ſuch, for example, as that which ſuppoſes the firſt germ to contain all the germs of the ſame ſpecies, or that every reproduction is a new creation, an immediate effect of the will of the Deity; for all hypo⯑theſes of this kind are mere matters of fact, con⯑cerning which it is impoſſible to reaſon. We muſt likewiſe reject every hypotheſis which is founded on final cauſes, ſuch as, that reproduc⯑tion is ordained in order to replace the living for the dead; that the earth may always be co⯑vered with vegetables and peopled with animals; that men may be ſupplied with abundance of nouriſhment, &c.; for ſuch hypotheſes, in place of explaining the effect by phyſical cauſes, ſtand on no other foundation than arbitrary relations and moral affinities. We ought, at the ſame time, to deſpiſe thoſe general axioms and phy⯑ſical problems ſo frequently and ſo injudiciouſ⯑ly [31] employed as principles by ſome philoſophers, ſuch as, `Nulla foecundatio extra corpus;' e⯑very living creature proceeds from an egg; ge⯑neration always ſuppoſes ſexes, &c. Theſe max⯑ims muſt not be taken in an abſolute ſenſe; they ſignify no more than that the thing happens more commonly in this manner than in any o⯑ther.

Let us, then, endeavour to find an hypotheſis that will be liable to none of theſe defects or in⯑cumbrances; and, if we ſhall not ſucceed in ex⯑plaining the mechaniſm employed by Nature for the reproduction of beings, we ſhall, at leaſt, be able to approach nearer to the truth than we have hitherto arrived.

In the ſame manner as we make moulds by which we can beſtow on the external parts of bodies whatever figure we pleaſe, let us ſuppoſe, that Nature can form moulds by which ſhe beſtows on bodies both an external and internal figure; would not this be one method by which reproduction might be effected?

Let us firſt conſider whether this ſuppoſition be well founded; let us examine whether it contains any thing that is abſurd or contradic⯑tory; and then we ſhall diſcover what conſe⯑quences may be drawn from it. Though our ſenſes reach not beyond the external parts of bodies, we have clear ideas of their different ſi⯑gures and external affections, and we can imi⯑tate Nature, by repreſenting external figures in [32] different ways, as by painting, by ſculpture, and by moulds. But, though our ſenſes be limited to external qualities, we know that bodies poſ⯑ſeſs internal qualities, ſome of which are general, as gravity. This quality or power acts not in proportion to the ſurfaces, but to the maſſes, or quantities of matter. Thus, there are in Nature powers, and even of the moſt active kind, which penetrate the internal parts of matter. We are unable to form diſtinct ideas of ſuch qualities; becauſe, not being external, they fall not under the cogniſance of our ſenſes. But we can com⯑pare their effects, and may draw analogies from them, in order to account for the effects of ſimi⯑lar qualities.

If our eyes, in place of repreſenting to us only the ſurfaces of bodies, were ſo conſtructed as to perceive their internal parts alone, we ſhould then have clear ideas of the latter, without knowing any thing of the former. Upon this ſuppoſition, moulds for the internal conſtitution, which I have ſuppoſed to be employed by Nature, would be equal⯑ly obvious and eaſy to conceive as moulds for the external figures of bodies; and we ſhould then be in a condition to imitate the internal parts of bodies, as we now imitate the external. Theſe internal moulds, though beyond our reach, may be in the poſſeſſion of Nature, as ſhe endows bodies with gravity, which penetrates every par⯑ticle of matter. The ſuppoſition of internal moulds being thus founded on analogy, let us [33] next examine whether it involves any contradic⯑tion.

It may be alledged, that the expreſſion, inter⯑nal mould, includes two oppoſite and contradic⯑tory ideas; for the idea of a mould relates only to the furface; but the idea of internal, as here em⯑ployed, has a relation to the whole maſs; and therefore we might, with equal propriety, talk of a maſſy furface as of an internal mould.

I allow, that, when ideas are attempted to be repreſented which have never been expreſſed, we are ſometimes obliged to uſe terms that are apparently contradictory. To avoid this incon⯑venience, philoſophers have been accuſtomed to employ unuſual terms, in place of thoſe which have a received ſignification. But this artifice is of no uſe, when we can ſhow, that the ſeem⯑ing contradiction lies in the words, and not in the idea. But a ſimple idea cannot include a contradiction; i. e. whenever we can form an idea of a thing, if this idea be ſimple, it cannot be complex; it can include no other idea; and, of courſe, it can contain nothing that is oppoſite or contradictory.

Simple ideas are not only the firſt apprehen⯑ſions received by the ſenſes, but the firſt com⯑pariſons which we form of theſe apprehenſions: For the firſt apprehenſion is always the reſult of compariſon. The idea of the largeneſs or di⯑ſtance of an object neceſſarily implies a compa⯑riſon with bulk or diſtance in general. Thus, [34] when a idea includes nothing more than com⯑pariſon, it ought to be regarded as ſimple; and, conſequently, it can contain nothing contradic⯑tory. The idea of an internal mould is of this ſpecies. There is in nature a quality known by the name of gravity, which penetrates the in⯑ternal parts of bodies. I underſtand the idea of an internal mould to be relative to gravity; and, therefore, as it includes only a compariſon, it can imply no contradiction.

Let us now trace the conſequences that may be drawn from this ſuppoſition; let us likewiſe inveſtigate ſuch facts as may correſpond with it; and the more analogies we can collect, the ſup⯑poſition will be rendered the more probable. We ſhall begin with unfolding the idea of external moulds, and how it may lead us to conceive the mode of reproduction.

Nature, in general, appears to have a greater bias towards life than death: She ſeems anxious to organize bodies as much as poſſible. Of this the multiplication of germs, which may be in⯑finitely increaſed, is a convincing proof; and it may be ſafely affirmed, that, if all matter is not organized, it is only becauſe organized beings deſtroy one another; for we can increaſe, at pleaſure, the number of animals and vegetables; but we cannot augment the quantity of ſtones or of dead matter. This appears to indicate, that the moſt ordinary and familiar operation of Na⯑ture [35] is the production of organized bodies; and here her power knows no limitation.

To render this more plain, we ſhall calculate what may be produced by a ſingle germ. The ſeed of an elm, which weighs not above the hun⯑dredth part of an ounce, will, in 100 years, form a tree, of which the maſs will amount to ten cubic fathoms. But, at the tenth year, this elm will have produced 1000 ſeeds, each of which, in 100 years more, will conſiſt of ten cubic fathoms. Thus, in the ſpace of 110 years, more than 10,000 cubic fathoms of organized matter are produced. Ten years after, we ſhall have ten million of fathoms, without including the annu⯑al increaſe of 10,000, which would amount to 100,000 more; and, in ten years more, the num⯑ber of cubic fathoms would be 10,000,000,000,000. Hence, in 130 years, a ſingle germ would pro⯑duce a maſs of organized matter equal to 1000 cubic leagues; for a cubic league contains only about 10,000,000,000 cubic fathoms; ten years after, this maſs would be increaſed to a thouſand times a thouſand leagues, or one million of cu⯑bic leagues; and, in ten more, it would amount to 1,000,000,000,000 cubic leagues; ſo that, in the ſpace of 150 years, the whole globe might be converted into organized matter of a ſingle ſpecies. Nature would know no bounds in the production of organized bodies, if her progreſs were not obſtructed by matter which is not ſuſ⯑ceptible [36] of organization; and this is a full de⯑monſtration that ſhe has no tendency to increaſe brute matter; that her ſole object is the multi⯑plication of organized beings; and that, in this operation, ſhe never ſtops but when irreſiſtible obſtacles occur. What we have remarked con⯑cerning the ſeed of an elm, may be extended to any other germ; and it would be eaſy to ſhow, that, by hatching all the eggs which are produ⯑ced by hens for a courſe of 30 years, the num⯑ber of fowls would be ſo great as to cover the whole ſurface of the earth.

Calculations of this kind evince the tenden⯑cy of Nature towards the production of orga⯑nized bodies, and the facility with which ſhe performs the operation. But I will not ſtop here. Inſtead of dividing matter into orga⯑nized and brute matter, the general diviſion ought to be into living and dead matter. That what is called brute matter is nothing but matter produced by the death of animals and vegetables, might be proven from the enormous quantities of ſhells, and other relicts of living bodies, which conſtitute the principal part of ſtones, marbles, clays, marles, earths, turfs, and other ſubſtances that are commonly reckoned brute matter, but are, in reality, compoſed of decayed animals and vegetables. This doctrine will be farther illuſtrated by the ſubſequent remarks, which appear to be well founded.

[37] The great facility and activity of Nature in the production of organized bodies, the exiſtence of infinite numbers of organic particles which conſtitute life, have been already demonſtrated. We now proceed to inquire into the principal cauſes of death and deſtruction. In general, beings which have a power of converting matter into their own ſubſtances, or of aſſimilating the parts of other beings, are the greateſt deſtroyers. Fire, for example, which converts almoſt every ſpecies of matter into its own ſubſtance, is the greateſt ſource of deſtruction that we are ac⯑quainted with. Animals ſeem to partake of the nature of flame; their internal heat is a ſpecies of fire next to flame. Accordingly, animals are the greateſt deſtroyers; and they aſſimilate and convert into their own ſubſtance all bodies which can ſerve them for nouriſhment. But, though theſe two cauſes of deſtruction be very conſide⯑rable, and their effects tend perpetually to the deſtruction of organized bodies, the cauſe of re⯑production is infinitely more active and power⯑ful. It even ſeems to derive, from deſtruction itſelf, freſh powers of multiplying; for aſſi⯑milation, which is one cauſe of death, is, at the ſame time, a neceſſary mean of producing life.

The deſtruction of organized bodies, as has been remarked, is only a ſeparation of the orga⯑nic particles of which they are compoſed. Theſe particles continue ſeparate till they be again u⯑nited [38] by ſome active power. But what is this power? It is the power, poſſeſſed by animals and vegetables, of aſſimilating the matter of their food; and is not this the ſame, or nearly con⯑nected with the ſame power which is the cauſe of reproduction?

CHAP. III.
Of Nutrition and Growth.

AN animal body is a kind of internal mould, in which the nutritive matter is ſo aſſi⯑milated to the whole, that, without changing the order or proportion of the parts, each part receives an augmentation. This increaſe of bulk has, by ſome philoſophers, been called an expanſion or unfolding of the parts; becauſe they fancied they had accounted for the phae⯑nomenon, by telling us, that the form of an animal in embryo was the ſame as at full matu⯑rity, and that, therefore, it was eaſy to conceive how its parts ſhould be proportionally unfolded and augmented by the addition of acceſſory matter.

But, how can we have a clear idea of this aug⯑mentation or expanſion, if we conſider not the bodies of animals, and each of their parts, as ſo many internal moulds which receive the acceſ⯑ſory matter in the order that reſults from their poſition and ſtructure? This expanſion cannot be effected ſolely by an addition to the ſurfaces, but, on the contrary, by an intus-ſuſception, or by penetrating the whole maſs; for the ſize of [40] the part is augmented proportionally, without changing its form. Hence it is neceſſary, that the increaſing matter muſt, in ſome manner or other, intimately penetrate the whole part in all its dimenſions: It is equally neceſſary, that this penetration ſhould be effected in a fixed order and proportion, ſo that no internal point receive more matter than another; otherwiſe ſome parts would be more quickly unfolded than others, which would entirely change their figure. What can thus regulate the acceſſory matter, and force it to arrive equally and proportionally to every internal point of the body, if we have not re⯑courſe to an internal mould?

The bodies of animals and of vegetables, there⯑fore, conſiſt of internal moulds, which uniform⯑ly preſerve the ſame figure. But their maſſes may receive a proportional increaſe, by the ex⯑panſion of the moulds in all their dimenſions, both internal and external; and this expanſion is effected by an intus-ſuſception of an acceſ⯑ſory and foreign matter, which intimately pe⯑netrates the whole, and aſſumes the ſame form and identity of ſubſtance with the matter of the moulds themſelves.

But what is the nature of that matter which an animal, or a vegetable, aſſimilates to its own ſubſtance? What beſtows on it that force and activity which enables it to penetrate the inter⯑nal mould? If ſuch a power exiſts, muſt it not [41] be ſimilar to that by which the mould itſelf is capable of being reproduced?

Theſe three queſtions include the whole ſub⯑ject, and appear to depend on one another; for it is impoſſible to explain, in a ſatisfactory man⯑ner, the reproduction of animals or vegetables, if we have not a clear idea how the operation of nutrition is performed. Each queſtion, there⯑fore, demands a ſeparate examination, that we may be enabled to compare their reſults.

The firſt, which regards the nature and qua⯑lities of the nutritive matter, is in part reſolved by the preceding reaſonings, and ſhall be clear⯑ly unfolded in the ſubſequent chapters. We ſhall demonſtrate, that there are in nature infi⯑nite numbers of living organic particles; that nature produces them without any expence, becauſe their exiſtence is conſtant and invariable; that the cauſes of death only diſunite theſe par⯑ticles, but do not deſtroy them. Thus the mat⯑ter aſſimilated by an animal or vegetable, is an organic matter of the ſame nature with that of the animal or vegetable, and, conſequently, may augment the ſize without changing the figure or the qualities of the original moulds; becauſe it has the ſame qualities and the ſame form with the matter of which the moulds themſelves are compoſed. Of the quantity of aliment taken by an animal to ſupport its life, and to maintain the vigour of its organs, and of the juices ab⯑ſorbed by the roots and leaves of a plant, a great [42] part is rejected by tranſpiration, by ſecretions, and by other excretories; and a ſmall portion only is retained for the nouriſhment and expan⯑ſion of the parts. It is extremely probable, that, in the bodies of animals and of vegetables, a ſe⯑paration is made between the brute particles of the aliment and the organic; that the former are carried off by the methods juſt mentioned; that nothing but the organic particles remain; and that they are diſtributed, by means of ſome active power, to the different parts, in a proportion ſo exact, that neither more nor fewer are applied than anſwer the purpoſes of nutrition, and of an equal growth and expanſion.

As to the ſecond queſtion, What is the nature of that active power, which enables the organic matter to penetrate and combine with the inter⯑nal mould? It is apparent, from the preceding chapter, that powers exiſt in nature, like that of gravity, which affect the moſt internal parts of matter, without having the ſmalleſt relation to its external qualities. Theſe powers, as former⯑ly obſerved, are beyond the reach of our ſenſes; becauſe their action is exerted upon the intimate ſtructure of bodies. It is evident, therefore, that we can never obtain a clear idea of them, nor of their mode of acting. Their exiſtence, how⯑ever, is not leſs certain, than that, by means of them, moſt natural effects are produced, eſpe⯑cially thoſe of nutrition and expanſion, which muſt be owing to a cauſe which penetrates the [43] moſt intimate receſſes of the original moulds; for, in the ſame manner as gravity pervades the whole parts of matter, the power which puſhes forward or attracts the organic particles of food, penetrates the internal parts of organized bodies; and, as theſe bodies have a certain form, which we have diſtinguiſhed by the appellation of in⯑ternal moulds, the organic particles, puſhed on by the action of this penetrating force, muſt en⯑ter in an order relative to this form, and conſe⯑quently cannot alter its figure, but only augment its bulk, and give riſe to the growth and expan⯑ſion of organized bodies: And if, in the orga⯑nized body, thus expanded, there be ſome par⯑ticles ſimilar to the whole, both internally and externally, theſe parts will become the ſource of reproduction.

Let us now examine the third queſtion, name⯑ly, Is it not by a ſimilar power that the internal mould itſelf is reproduced? This power appears to be not only ſimilar, but the very ſame with that which is the cauſe of expanſion and reproduc⯑tion; for, in an organized and expanded body, nothing farther is neceſſary for the reproduction of a new body ſimilar to itſelf, than that it ſhould contain ſome particle every way ſimilar to the whole. This particle, at its firſt ſeparation, will not preſent to our eyes a ſenſible figure by which we can compare it with the whole body. But, when ſeparated from the body, and put in a ſi⯑tuation to receive proper nouriſhment, this ſimi⯑lar [44] particle will begin to expand and to exhibit the form of an entire and independent being, of the ſame ſpecies with that from which it was detached. Thus, a willow or a polypus, as they contain a larger proportion of particles ſimilar to the whole, than moſt other ſubſtances, when cut into any indefinite number of pieces, each ſegment becomes a new body ſimilar to the pa⯑rent from which it was ſeparated.

Now, in a body of which all the particles are ſimilar to itſelf, the organization is the moſt ſimple, as has been remarked in the firſt chap⯑ter; for it is only a repetition of the ſame form, a congeries of figures ſimilarly organized. It is for this reaſon that the moſt ſimple bodies, the moſt imperfect ſpecies, are moſt eaſily and moſt abundantly reproduced. But, if an organized body contain only few particles ſimilar to itſelf, as theſe alone are capable of a ſecond expanſion, its power of reproducing will be both more dif⯑ficult, and more circumſcribed as to the number produced. The organization of bodies of this laſt kind is alſo more complex, becauſe it poſ⯑ſeſſes fewer parts which are ſimilar to the whole; and, therefore, the more perfectly a body is or⯑ganized, its power of reproduction will be pro⯑portionally diminiſhed.

In this manner we diſcover nouriſhment, growth, and propagation, to be effects of the ſame cauſe. Organized bodies are nouriſhed by the particles of aliment which are ſimilar to them; [45] they grow or are expanded by abſorbing thoſe or⯑ganic particles which correſpond to their own na⯑ture; and they propagate, becauſe they contain ſome organic particles ſimilar to themſelves. It only remains to examine whether theſe ſimilar organic particles are extracted from the food, or have a primary and independent exiſtence in the bodies themſelves. If we ſuppoſe the latter, we recur to the infinity of ſimilar parts or germs contained within each other, an hypotheſis which we have already demonſtrated to be replete with difficulties and abſurdities. We muſt, therefore, hold, that the ſimilar parts are extracted from the food; and, after what has been ſaid on the ſub⯑ject, we hope to be able to explain the manner of their abſorption, and how the more minute organic particles which compoſe them are u⯑nited.

We formerly remarked, that the organic parts of food were ſeparated from thoſe which have no analogy to the animal or vegetable, by tran⯑ſpiration and other excretions. The firſt re⯑main, and ſerve to expand and nouriſh the body: But theſe organic parts muſt be of very differ⯑ent ſpecies; and, as each part of the body re⯑ceives only a proper number of thoſe which corre⯑ſpond to it, the ſurplus, it is natural to imagine, will be returned from all parts of the body, and be collected in one or more reſervoirs, where they will unite and form ſmall organic bodies ſimilar to the firſt, and which require nothing but pro⯑per [46] circumſtances for expanding and becoming new individuals of the ſame ſpecies; for, all parts of the body ſending off organic particles ſimilar to thoſe of which themſelves are compoſed, the reſult of their union muſt be the production of new organized bodies ſimilar to the original. This being granted, may we not conclude, that this is the reaſon why organized bodies, during the time of their growth and expanſion, are ſel⯑dom or never capable of reproducing; becauſe the growing parts abſorb the whole organic par⯑ticles preſented to them, and no ſurplus being ſent from the different parts of the body, pro⯑pagation becomes, of courſe, impracticable?

This account of nutrition, and of reproduc⯑tion, will not, perhaps, be received by thoſe phi⯑loſophers who admit only a certain number of mechanical principles, and reject every thing as falſe which depends not upon them; and, as the explication now given of nutrition and repro⯑duction has no connection with any of theſe principles, they will conclude that it deſerves no credit. But I think very differently from theſe philoſophers. In admitting only a few mechanical principles, they conſider not how much they contract the bounds of philoſophy, and how few phaenomena can, by this narrow method of thinking, be fully expiſcated.

The notion of explaining all the appearances in nature upon the principles of mechaniſm, is, doubtleſs, a great exertion, and was firſt attempt⯑ed [47] by Des Cartes. But it is, at leaſt, an unte⯑nible project; and, though it were otherwiſe, we are unable to put it in execution. Theſe mechanical principles are, the extenſion of mat⯑ter, its impenetrability, its motion, its external figure, its diviſibility, the communication of mo⯑tion by impulſe, by the action of ſprings, &c. Theſe ideas we have acquired by our ſenſes, and we regard them as principles, becauſe they are general and common to all matter. But are we certain that matter poſſeſſes no other qualities? Ought we not rather to believe that theſe qua⯑lities, which we aſſume for principles, are only modes of perception; and that, if the confor⯑mation of our ſenſes were different, we would recognize qualities in matter very different from thoſe above enumerated? It is preſumptuous to deny every quality to matter but thoſe we are acquainted with. Many general qualities, per⯑haps, remain to be diſcovered; and many may exiſt which will for ever elude human diſcern⯑ment. The cauſe of impulſion, of coheſion, or of any other mechanical principle, will always continue to be equally inſcrutable as that of at⯑traction, or of any other general quality. Hence it may be concluded, that mechanical principles are nothing elſe than general effects which ex⯑perience has enabled us to remark in matter; and that, whenever we ſhall diſcover, either by reflection, by analogy, or by experience, a new general effect, it will become a new mechanical [48] principle, which may be employed with equal advantage and certainty as any of thoſe that are already known.

The defect of Ariſtotle's philoſophy was the employing particular effects as cauſes; and that of Des Cartes conſiſts in the rejection of every cauſe, but a few general effects. To uſe nothing as cauſes but general effects, to endeavour to augment the number of theſe, and to attempt to generalize particular effects, would conſtitute the moſt perfect principles of genuine philoſophy.

In my theory of expanſion and reproduction, I firſt admit the mechanical principles, then the penetrating force of gravity, and, from analogy and experience, I have concluded the exiſtence of other penetrating forces peculiar to organized bodies. I have proved by facts, that matter has a ſtrong tendency towards organization; and that there are in nature an infinite number of organic particles. I have, therefore, only gene⯑ralized particular obſervations, without advan⯑cing any thing contrary to mechanical principles, when that term is uſed in its proper ſenſe, as de⯑noting the general effects of Nature.

CHAP. IV.
Of the Generation of Animals.

AS the organization of men, and of other animals, is the moſt perfect, and the moſt complex, the propagation of them is like⯑wiſe moſt difficult, and the number of indivi⯑duals is leſs abundant. I except here ſuch ani⯑mals as can be multiplied by a ſeparation of their parts, or without the aid of generation, theſe having been ſufficiently treated of in the pre⯑ceding chapter^*.

But how will the theory delivered in the former chapter apply to the generation of men, and other animals, who are diſtinguiſhed by ſexes? We underſtand, from what has been ſaid, how every individual may reproduce; but we cannot conceive how two individuals, the one a male, and the other a female, ſhould uniformly produce a third.

Before replying to this objection, I muſt ob⯑ſerve, that the writers on this ſubject have con⯑fined their ideas ſolely to the generation of men and of animals, without attending to the nature [50] of reproduction in general: And, as the generation of animals is the moſt complicated ſpecies of re⯑production, they have laboured under great diſ⯑advantages, not only by attacking the moſt diffi⯑cult point, but by leaving themſelves no ſubject of compariſon to enable them to illuſtrate the queſtion. To this circumſtance I chiefly attri⯑bute the unſucceſsfulneſs of their attempts. But, by the route I have taken, I am perſuaded that I ſhall be able to give a ſatisfactory explanation of every ſpecies of reproduction.

Let the generation of man ſerve as an ex⯑ample. To begin with infancy:

The expanſion and growth of the different parts of man's body being effected by the inti⯑mate penetration of organic particles analogous to each of theſe parts, all the organic particles, in early life, are abſorbed, and entirely employed in unfolding and augmenting his different mem⯑bers. He has, of courſe, little or no ſuperflu⯑ous particles, till his growth be completed. It is for this reaſon that infants are incapable of pro⯑pagating. But, when his body has nearly at⯑tained its full ſize, he requires not the ſame quantity of organic particles; the ſurplus is, therefore, ſent from all parts into reſervoirs de⯑ſtined for their reception. Theſe reſervoirs are the teſtes and ſeminal veſſels. At this very pe⯑riod, when the growth of the body is nearly finiſhed, puberty commences, and every phae⯑nomenon attending it diſcovers a ſuperabundance [51] of nouriſhment: The voice changes into a deeper tone; the beard begins to appear, and other parts of the body are covered with hair; the parts deſtined for generation are ſuddenly expanded; the ſeminal fluid fills the reſervoirs prepared for its reception; and, without pro⯑vocation, it eſcapes from the body during ſleep. This ſuperabundance is ſtill more evident in the female: It diſcovers itſelf by a periodic evacua⯑tion, which begins and terminates with the fa⯑culty of propagating; by a quick increaſe of the breaſts; and by a change in the ſexual parts, which ſhall be afterwards explained^*.

I conceive, then, that the organic particles ſent from all parts of the body into the teſticles and ſeminal veſſels of the male, and into the ovarium of the female, compoſe the ſeminal fluid, which, in either ſex, as formerly obſerved, is a kind of extract from the ſeveral parts of the body. Theſe organic particles, inſtead of uni⯑ting and forming an individual ſimilar to that in whoſe body they are contained, as happens in vegetables, and ſome imperfect animals, cannot accompliſh this end without a mixture of the fluids of both ſexes. When this mixture is made, if the organic particles of the male exceed thoſe of the female, the reſult is a male; and, if thoſe of the female moſt abound, a female is gene⯑rated. I mean not that the organic particles of the male or of the female could ſingly produce [52] individuals: A concurrence or union of both is requiſite to accompliſh this end. Thoſe ſmall moving bodies, called ſpermatic animals, which, by the aſſiſtance of the microſcope, are ſeen in the ſeminal fluids of all male animals, are, per⯑haps, organized ſubſtances proceeding from the individual which contains them; but, of them⯑ſelves, they are incapable of expanſion, or of becoming animals ſimilar to thoſe in whom they exiſt. We ſhall afterwards demonſtrate, that there are ſimilar animalcules in the ſeminal fluids of females, and point out the place where this fluid is to be found.

It is probable, that theſe organic bodies are only the firſt rudiments of an animal, containing nothing but its eſſential parts. We ſhall not en⯑ter into a detail of proofs on this ſubject, but content ourſelves with remarking, that the or⯑ganization of theſe pretended ſpermatic animals may be very imperfect; or rather, that they have the living organic particles mentioned above, which are common both to vegetables and to animals; or, at moſt, that they are only the firſt junction of theſe particles.

But, to return to our ſubject. It may be aſk⯑ed, how is it poſſible that the ſuperfluous orga⯑nic particles ſhould be detached from all parts of the body, and unite upon the mixture of the male and female fluids? Beſides, are we certain that ſuch a mixture takes place? Has it not been mentioned, that the female furniſhes no fluid of [53] this kind? Is it an eſtabliſhed fact, that the male fluid enters the uterus? &c.

To the firſt queſtion I reply, that, if what I had ſaid concerning the penetration of the in⯑ternal mould by the organic particles, in growth and nutrition, had been properly underſtood, it would be eaſy to conceive, that, when theſe particles are unable to penetrate the parts into which they formerly entered, they muſt take another rout, and, of courſe, arrive at ſome other part, as the teſticles and ſeminal veſſels. Every attempt to explain the animal oeco⯑nomy, and the various motions of the human body, by mechanical principles alone, muſt be vain and ineffectual; for it is evident, that the circulation of the blood, muſcular motion, and other functions of an animated body, cannot be accounted for by impulſion, or by any of the common laws of mechaniſm. It is equally evi⯑dent, that growth and reproduction are effects of laws of a different nature. Why, then, do we refuſe the exiſtence of penetrating forces which act upon the whole ſubſtances of bodies, when we have examples of ſuch powers in gra⯑vity, in magnetic attraction, in chemical affini⯑ties? Since, therefore, we are aſſured by facts, and by a number of conſtant and uniform ob⯑ſervations, that there are powers in nature which act not by impulſion, why are not theſe powers ranked among mechanical principles? Why do we reject them in the explanation of effects [54] which they are known to produce? Why are we deſirous of employing only the power of impul⯑ſion? Is not this equally abſurd as to judge of painting by the touch; to explain the phaeno⯑mena that belong to the maſs by thoſe that relate only to the ſurface; or to uſe one ſenſe in place of another? It is limiting the reaſoning faculty to a ſmall number of mechanical principles, which are by no means ſufficient to explain the various effects of nature.

But, if theſe penetrating forces be once ad⯑mitted, is it not natural to imagine, that thoſe par [...]es which are moſt analogous to one ano⯑ther will unite in the moſt intimate manner; that each part of the body will appropriate thoſe which are moſt agreeable to its nature; and that the whole ſuperfluous particles will form a ſemi⯑nal fluid, which ſhall contain all the organic [...]r⯑ticles neceſſary for forming a ſmall organized body, ſimilar in every reſpect to that from which the fluid is extracted? May not a force, ſimilar to that which is the cauſe of growth, be ſufficient to collect the ſuperfluous organic particles, and beſtow on them the figure of the body from which they proceed?

That our food contains an immenſe number of organic particles, requires no formal proof; ſince we are ſolely nouriſhed by animals and ve⯑getables, which are organized ſubſtances. In the ſtomach and inteſtines, the groſs parts of the a⯑liment are ſeparated and rejected by the excre⯑tories. [55] The chyle, which is a purer part of the aliment, is abſorbed by the lacteal veſſels; from thence it is carried into the maſs of blood, and, in the courſe of circulation, it is more and more refined, the unorganic and uſeleſs particles being thrown out by tranſpiration and other ſe⯑cretions: But the organic particles are retained, becauſe they are analogous to the blood, and are attracted by it. Hence, as the whole maſs of blood paſſes ſeveral times through the body, during the courſe of this perpetual circula⯑tion, I ſuppoſe, that each particular part attracts thoſe particles which are moſt analagous to it, and allows the reſt to move on. In this manner all the parts are nouriſhed and unfolded, not, as is commonly imagined, by a ſimple addition of matter to their furfaces, but by an intimate pene⯑tration of ſubſtance, effected by a force which acts equally upon every point of the whole maſs: And, after the different parts have acquired their utmoſt growth, and are fully impregnated with ſimilar organic particles, as their ſubſtance becomes then more denſe and ſolid, I imagine that they loſe their faculty of attracting and re⯑ceiving the particles preſented to them. But, as the particles continue to be carried round in the courſe of the circulation, and are no longer ab⯑ſorbed in ſuch quantities as formerly, they muſt, of neceſſity, be depoſited in ſome particular re⯑ſervoir, ſuch as the teſticles and ſeminal veſſels. When this fluid extract of the male is mixed [56] with that of the female, the particles which are moſt analogous to each other, being actuated by a penetrating force, unite and form a ſmall or⯑ganized body, ſimilar to the one or the other ſex; and this body, when once formed, requires only an expanſion of its parts, an operation which is performed in the womb of the mother.

We ſhall now conſider the ſecond queſtion, namely, whether the female has a ſeminal fluid ſimilar to that of the male? In the firſt place, though ſuch a fluid exiſts in females, the mode of emiſſion is very different from that of the male, being generally confined within the body^*. The antients were ſo confident of the exiſtence of a female fluid, that they diſtinguiſhed the two ſexes by their different modes of emiſſion. But thoſe phyſicians who attempt to explain genera⯑tion by eggs, or by ſpermatic animalcules, inſiſt, that females have no peculiar fluid; that the mucus iſſuing from the parts has been miſtaken for a ſeminal fluid; and that the opinion of the antients on this ſubject is deſtitute of foundation. This fluid, however, does exiſt; and the doubts concerning it have ariſen ſolely from attachment to ſyſtems, and from the difficulty of diſcover⯑ing its reſervoir. The fluid which is ſeparated from the glands about the neck and orifice of the uterus, has no viſible reſervoir; and, as it flows out of the body, it is natural to think that it is [57] not the prolific fluid, becauſe it cannot co-ope⯑rate in the formation of the foetus, which is performed within the uterus. The reſervoir for the prolific fluid of the female, therefore, muſt be ſitu⯑ated in a different part: It even flows abundant⯑ly; though, like that of the male, a ſmall quan⯑tity is ſufficient to produce a foetus. If a little of the male fluid enters the uterus, either by its orifice or by abſorption, and meets with the ſmalleſt drop of the female fluid, it is ſufficient for the purpoſe of propagation. Thus, nei⯑ther the obſervations of ſome anatomiſts, who maintain that the ſeminal fluid of the male can have no admiſſion into the uterus, nor the oppo⯑ſite opinion maintained by their antagoniſts, have any influence upon the theory we are en⯑deavouring to eſtabliſh. But the diſcuſſion of theſe points we leave to a future opportunity.

Having obviated ſuch objections as might be made, let us attend to the evidences that concur in ſupporting our hypotheſis. The firſt ariſes from the analogy between growth and repro⯑duction. It is impoſſible to give a ſatisfactory account of growth or expanſion, without having recourſe to thoſe penetrating forces, thoſe affini⯑ties or attractions which we employed in explain⯑ing the formation of the ſmall organic bodies that are ſimilar to the large bodies which con⯑tain them. A ſecond analogy is derived from this circumſtance, that both nutrition and repro⯑duction proceed, not only from the ſame effi⯑cient, [58] but from the ſame material cauſe, namely, the organic particles of food; and what proves the ſurplus of the nutritive particles to be the cauſe of reproduction, is, that the body is not in a condition to propagate till its growth be finiſh⯑ed: Of this we have daily examples, in dogs and other animals, who follow, more cloſely than we do, the laws of nature: They have no in⯑clination to propagate till they have nearly at⯑tained their full growth; and by this we know whether the growth of a dog be finiſhed; for he ſeldom grows after being in a condition to generate.

Another proof that the ſeminal fluid is form⯑ed of the ſurplus of the nutritive particles, ariſes from the condition of eunuchs and other muti⯑lated animals: In this unnatural ſtate, animals grow fatter than thoſe who retain all their parts. The ſuperabundance of nutriment, having no organs for its evacuation, changes the whole ha⯑bit of their bodies. The knees and haunches of eunuchs grow uncommonly large. The rea⯑ſon is evident. After their bodies have acquired the common ſize, if the ſuperfluous organic par⯑ticles found an iſſue, as in other men, the growth would proceed no farther. But, as they want organs for emitting the ſeminal fluid, which is nothing but the ſuperfluous nutritive particles, it remains in the body, and has a conſtant ten⯑dency to expand the parts beyond their natural ſize. Now, bones, it is well known, grow or [59] extend by their extremities, which are ſoft and ſpongy, and, when they have once acquired ſo⯑lidity, they are incapable of farther extenſion: Hence the ſuperfluous organic particles can on⯑ly enlarge the ſpongy extremities of bones; and this is the reaſon why the haunches, knees, &c. of eunuchs augment to a diſproportioned bulk.

But the ſtrongeſt proof of the truth of our preſent doctrine ariſes from the reſemblance of children to their parents. Sons, in general, reſemble their fathers more than their mothers, and daughters have a greater reſemblance to their mothers than their fathers; becauſe, with regard to the gene⯑ral habit of body, a man reſembles a man more than a woman, and a woman reſembles a wo⯑man more than a man. But, as to particular features or habits, children ſometimes reſemble the father, ſometimes the mother, and ſometimes both. A child, for example, will have the eyes of the father, and the mouth of the mother, or the colour of the mother and the ſtature of the father. Of ſuch phaenomena it is impoſſible to give any explication, unleſs we admit that both parents have contributed to the formation of the child, and, conſequently, that there has been a mixture of two ſeminal fluids.

Theſe reſemblances long embarraſſed me, and, till I had maturely conſidered the ſubject of ge⯑neration, led me into many errors and prejudices: And it was not without much thought, a minute [60] examination of a great number of families, and a multiplicity of evidence, that I could prevail on myſelf to alter my former opinion, and to em⯑brace what I now believe to be truth. But the objections which might occur concerning mulat⯑toes, mongrels, mules, and particular parental reſemblances, inſtead of oppoſing my theory, I deſpair not of being able to ſhow that they ſtrengthen and confirm it.

In youth, the ſeminal fluid is leſs copious, but more ſtimulating. Its quantity continues to augment till a certain age; becauſe, in propor⯑tion as we approach that age, the parts of the body become more ſolid, admit fewer nutritive particles, ſend back more of them to the com⯑mon reſervoirs, and, of courſe, augment the quantity of the ſeminal fluid. Thus, if the ex⯑ternal organs have not been uſed, middle-aged men, or even old men, procreate with more eaſe than young men. This is evidently the caſe with the vegetable tribes: A tree, the older it is, produces the greater quantity of fruit.

Young people, who, by forced irritations, de⯑termine an unnatural quantity of this fluid into the reſervoirs prepared for its reception, imme⯑diately ceaſe to grow, loſe their fleſh, and at laſt fall into conſumptions. The reaſon is apparent: They loſe, by premature and too frequent eva⯑cuations, the very ſubſtance which nature in⯑tended for the nouriſhment and growth of their bodies.

[61] Men who are thin, but not emaciated, and thoſe who are plump, but not fat, are the moſt vigorous. Whenever the ſuperabundant nutri⯑tive particles begin to form fat, it is always at the expence of the ſeminal fluid and other ge⯑nerative powers. When the growth of the dif⯑ferent parts of the body is compleat, when the bones have acquired full ſolidity, when the car⯑tilages begin to oſſify, and, laſtly, when the parts almoſt refuſe the admiſſion of nutritive particles, then the fat augments conſiderably, and the quantity of ſeminal fluid diminiſhes; becauſe the nutritive particles, inſtead of being ſent back to the reſervoirs, are arreſted in every part of the body.

The quantity of ſeminal fluid not only in⯑creaſes till we arrive at a certain age, but it be⯑comes more thick. It contains, in the ſame bounds, a greater quantity of matter. Its ſpecific gravity is nearly double that of the blood; and, of courſe, it is heavier than any o⯑ther animal fluid.

To a man in health, an evacuation of this fluid whets the appetite: He ſoon finds the ne⯑ceſſity of repairing the loſs by freſh nouriſh⯑ment. Hence we may conclude, that abſtinence and hunger are the moſt effectual checks to lux⯑ury of every kind.

Many other remarks might be made on this ſubject, which muſt be deferred till we come to treat of the hiſtory of man: We ſhall, therefore, [62] conclude with a few obſervations. Moſt ani⯑mals diſcover no inclination for the ſexes till their growth be nearly finiſhed: Thoſe which have but one ſeaſon in the year, have no ſemi⯑nal fluid excepting at that time. Mr Needham^* not only ſaw this liquor forming in the milt of the Calmar, but likewiſe the ſpermatic animals, and the milt itſelf, which have no exiſtence till the month of October, when this fiſh ſpawns on the coaſts of Portugal, where Needham made the obſervation. After the ſpawning time is over, the ſeminal liquor, the ſpermatic animals, and the milt, dry up and totally diſappear; till the ſame ſeaſon returns next year, when the ſuperfluous nutritive particles renew the milt as formerly. The hiſtory of the deer will furniſh us with an opportunity of remarking the various effects of rutting, the moſt conſpicuous of which is the extenuation of the animal; and, in thoſe ſpecies of animals whoſe rutting or ſpawning happens but once in a year, the extenuation of their bodies is proportionally great.

As women are ſmaller and weaker than men, as their conſtitutions are more delicate, and, as they take leſs food, it is natural to think that their ſuperfluous organic particles ſhould alſo be leſs abundant: Of courſe, their ſeminal fluid will be weaker and ſmaller in quantity than that of men; and, ſince the fluid of females contains fewer organic particles, muſt not a greater num⯑ber [63] of males than of females reſult from a mix⯑ture of theſe two fluids? This is really the caſe; and to account for it has hitherto been deemed impoſſible. The number of males born exceeds that of females about a ſixteenth part; and we ſhall afterwards ſee that the ſame effect is pro⯑duced by the ſame cauſe in all the different ſpe⯑cies of animals.

CHAP. V.
Examination of the different Syſtems of Generation.

PLATO, in the Timaeus, accounts not only for the generation of men, of animals, of plants, and of the elements, but even of the hea⯑vens and of the gods themſelves, by images reflected or extracted from the divine Creator; which images, by an harmonic movement, are arranged in the moſt perfect order, according to the properties of number. The univerſe, he ſays, is a copy of the Deity; time, ſpace, mo⯑tion, and matter, are the images or repreſenta⯑tions of his attributes; and ſecondary and parti⯑cular cauſes are reſults of the numeric and har⯑monic qualities of theſe images: The world, from its excellency, is the moſt perfect animated being. To give the world complete perfection, it was neceſſary that it ſhould contain all the o⯑ther animals, or all the poſſible forms and repreſentations of the creative power. Man is one of theſe forms. The eſſence of all genera⯑tion conſiſts in the unity and harmony of the number Three, or of the Triangle, namely, that which generates, that in which generation is performed, and the reſult, or that which is generated. The ſucceſſion of individuals in the [65] ſpecies, is only a fugitive image of the immu⯑table eternity of this harmonic triangle, a uni⯑verſal prototype of all exiſtences, and of all ge⯑nerations.

This philoſopher paints only ideas. Diſen⯑gaged from matter, he flies into the regions of abſtraction; and, loſing ſight of ſenſible objects, he contemplates thoſe of intellect alone. One cauſe, one end, one mean, compoſe the whole of his perceptions: God is the cauſe, perfection the end, and harmonic repreſentations the means. This idea is ſublime; the mode of philoſophi⯑ſing is noble and full of ſimplicity; but it is perfectly vacant, and affords no objects for ſpe⯑culation. We are not pure intelligences. We are unable to give real exiſtence to our ideas. Chained to matter, or rather depending on the cauſes of our ſenſations, it is impoſſible that we ſhould realiſe abſtractions. To Plato I might reply in his own manner, 'The Creator realiſes every thing he conceives; his perceptions be⯑get exiſtence: The created being, on the con⯑trary, conceives nothing but by retrenching from reality; and annihilation is neceſſary to bring forth his ideas.'

Let us, without regret, therefore, confine our⯑ſelves to a philoſophy more humble and more material; and, keeping within the ſphere which nature has allotted us, let us examine thoſe rapid and daring ſpirits who attempt, though in vain, to fly beyond the limits of humanity. The whole [66] of this Pythagorean philoſophy, which is purely intellectual, depends upon two principles, the one falſe, and the other uncertain; namely, the real power of abſtraction, and the natural exiſt⯑ence of final cauſes. To apprehend numbers to be real beings; to ſay that unity is a general individual, which not only repreſents all indivi⯑duals, but even communicates exiſtence to them; to pretend that unity exerciſes the actual power of engendering another unity nearly reſembling itſelf, and of creating two individuals, two ſides of a triangle, that can have no connection or perfection without a third ſide, which is neceſ⯑ſarily produced by the other two; in fine, to re⯑gard numbers, geometrical lines, and metaphy⯑ſical abſtractions, as real and efficient phyſical cauſes, by which the elements are formed, plants and animals regenerated, and all the phaeno⯑mena of nature produced, appears to be the greateſt and moſt abſurd abuſe of human rea⯑ſon, and an invincible obſtacle to the advance⯑ment of knowledge. Beſides, nothing can be more fallacious than ſuch chimeras. Sup⯑poſing we ſhould agree with Plato and Mal⯑branche, that matter has no exiſtence, that ex⯑ternal objects are only ideal images of the crea⯑tive power, and that we ſee every thing in the Deity himſelf; does it follow, that our ideas are of the ſame order with thoſe of the Creator, and that they can produce real exiſtences? Are we not dependent on our ſenſations? Whether the [67] objects which excite ſenſations be real or imagi⯑nary, whether they exiſt without or within, whe⯑ther it be God or matter that we every where behold, is to us of little importance: We are not leſs certain of being uniformly affected in the ſame manner by the ſame cauſes. The re⯑lations between our ſenſes and the objects which affect them, are neceſſary and invariable. It is upon this baſis alone that the principles of phi⯑loſophy ought to be founded, otherwiſe our knowledge muſt be uſeleſs and fallacious. Can an harmonic triangle create the ſubſtance of the elements? Is fire, as Plato affirms, an acute tri⯑angle, and light and heat two properties of this triangle? Are water and air rectangular and e⯑quilateral triangles? Is the form of the element of earth a ſquare, becauſe, being the leaſt per⯑fect of the four elements, it recedes as far as poſſible from a triangle, without departing alto⯑gether from its eſſence? Do males and females embrace each other, for no other purpoſe but to complete the triangle of generation? Theſe Pla⯑tonic ideas have two different aſpects: In ſpe⯑culation, they ſeem to proceed from ſublime principles; but the application of them in prac⯑tice leads to nothing but falſe and childiſh con⯑cluſions.

Is it difficult to perceive that our ideas origi⯑nate from our ſenſes alone; that the objects we regard as real exiſtences, are thoſe concerning which the ſenſes uniformly give the ſame teſti⯑mony; [68] that the objects we apprehend as having a real exiſtence, are thoſe which are invariably preſented to us in the ſame manner; that the mode in which they preſent themſelves has no dependence upon our will or inclination; that, of courſe, our ideas, inſtead of being the cauſes of things, are only particular effects, which be⯑come leſs ſimilar to the objects themſelves, in proportion as they are rendered more general; and, laſtly, that mental abſtractions are only ne⯑gative beings, which derive their intellectual ex⯑iſtence from the faculty we poſſeſs of conſider⯑ing objects, without regarding their ſenſible qua⯑lities

Is it not, therefore, apparent, that abſtract i⯑deas can never be the principles of exiſtence, or of real knowledge? On the contrary, all our knowledge is derived from comparing and ar⯑ranging the reſults of our ſenſations. Theſe re⯑ſults are known by the appellation of experience, the only ſource of genuine ſcience. The em⯑ployment of any other principle is an abuſe; and every edifice founded upon abſtract ideas, is a temple erected to Error.

In philoſophy, error has a more extenſive in⯑fluence than in morals. A thing may be falſe in morals ſolely becauſe it is miſrepreſented. But falſehood in metaphyſics conſiſts not in miſre⯑preſentation alone, but in taking for granted what has no exiſtence at all. It is into this moſt per⯑nicious ſpecies of error that the Platoniſts and [69] the Sceptics have fallen. Their falſe ſuppoſitions have obſcured the natural light of truth, bewil⯑dered the reaſoning faculties of men, and retard⯑ed the progreſs of philoſophy.

Final cauſes are employed as a ſecond prin⯑ciple by Plato and other theoriſts. This prin⯑ciple has even been adopted by the vulgar, and by ſome modern philoſophers. A moment's re⯑flection, however, will be ſufficient to reduce this principle to its proper value. To ſay that light exiſts becauſe we have eyes, and that ſounds exiſt becauſe we have ears; or to ſay that we have eyes and ears, becauſe light and ſounds exiſt; is not this preciſely the ſame thing? or, rather, are we any wiſer by this kind of reaſon⯑ing? Will we ever make any diſcoveries by ſuch a mode of explication? Is it not apparent, that final cauſes are only arbitrary relations and moral abſtractions, which ought to have leſs in⯑fluence than abſtractions in metaphyſics, becauſe the origin of the former is leſs noble and worſe imagined? And, though Leibnitz has endeavour⯑ed to give an elevation to final cauſes, under the appellation of the reaſonableneſs and eternal fit⯑neſs of things, [raiſon ſuffiſante], and Plato has repreſented them under the flattering picture of abſolute perfection; all theſe efforts are inſuffi⯑cient to cover their native inſignificance and precariouſneſs. Are we better inſtructed in the operations of Nature, becauſe we are told that nothing exiſts without a reaſon, or that every [70] thing is created with a view to the perfection of the whole? What is reaſonableneſs or fitneſs? What is perfection? Are they not moral beings, created ſolely by the human intellect? Are they not arbitrary relations which we have contrived to generaliſe? They have no foundation but in moral affinities, which, ſo far from producing any phyſical or real exiſtence, change the nature of truth, and confound the objects of our ſen⯑ſations, of our perceptions, and of our under⯑ſtandings, with thoſe of our ſentiments, of our paſſions, and of our wills^*.

[71] Much more might be ſaid upon this ſubject. But I pretend not to write a treatiſe on philo⯑ſophy; and ſhall therefore return to phyſics, from which the ideas of Plato, concerning uni⯑verſal generation, have diverted my attention. Ariſtotle, who was as great a philoſopher as Plato, and a better phyſician, inſtead of wan⯑dering in the regions of theory, collects facts, and ſpeaks in a language more intelligible.

Matter, he remarks, which is only a capacity of receiving forms, aſſumes, in generation, a fi⯑gure ſimilar to the individual which furniſhes it: And, with regard to animals which generate by the intervention of ſexes, he imagines, that the prolific principle proceeds ſolely from the male^*: For though, in another place, when ſpeaking of animals in general, he ſays, that the female [72] ſheds a ſeminal fluid within the body, it appears, that he regards not this fluid as a prolific prin⯑ciple; and yet he tells us, that the menſtrual blood ſerves for the formation, nouriſhment, and growth of the foetus; but that the efficient principle exiſts alone in the ſeminal fluid of the male, which acts not as matter, but as a cauſe. Averrhoes, Avicenna, and other philoſophers who embraced this opinion of Ariſtotle, have endea⯑voured to prove that females have no prolific fluid. They alledge, that, as females are fur⯑niſhed with a menſtrual fluid, which is both neceſſary and ſufficient for the purpoſes of ge⯑neration, it is unnatural to ſuppoſe them poſ⯑ſeſſed of any other, eſpecially ſince it begins to appear, like that of the male, at the age of pu⯑berty. Beſides, they continue, if females really have a prolific ſeminal fluid, why do they not produce without the intercourſe of the male, ſince they contain the prolific principle, as well as the matter neceſſary for the growth and ex⯑panſion of the embryo? This laſt reaſon is the only one which merits attention. The men⯑ſtrual blood appears to be neceſſary for the growth and nouriſhment of the foetus; but ſtill it may contribute nothing to its firſt formation, which requires the mixture of both prolific fluids. Females, therefore, like males, may have a prolific fluid for the formation of the embryo, as well as menſtrual blood for its growth and nouriſhment. The imagination is not unnatu⯑ral, [73] that, as the female poſſeſſes both a prolific fluid extracted from all parts of her body, and likewiſe the means of expanding and nouriſh⯑ing, ſhe ſhould produce females without any communication with the male. It muſt be al⯑lowed, that this metaphyſical argument uſed by the Ariſtotelians for proving that females are deſtitute of a prolific fluid, may be urged as the ſtrongeſt argument againſt every ſyſtem of ge⯑neration, and, in particular, againſt that which I am endeavouring to eſtabliſh.

Let us ſuppoſe, it may be ſaid, that the ſuper⯑fluous organic particles are ſent from every part of the body into the teſticles and ſeminal veſſels of the male, why do they not, by means of your imaginary attracting forces, form ſmall organi⯑zed bodies ſimilar to the whole? Why are not ſimilar bodies generated in the female, without any intercourſe with the male? If you anſwer, that the male fluid contains only males, that the female fluid contains only females, that both pe⯑riſh for want of the circumſtances neceſſary for expanſion, and that, for the procreation of an animal, a mixture of both is requiſite; may it not be demanded, why this moſt complicated, difficult, and leſs fertile mode of generation, is ſo invariably preferred by Nature, that all animals, with a few trifling exceptions, generate by the mutual commerce of ſexes?

I ſhall content myſelf, at preſent, with reply⯑ing, that this is the mode actually employed by [74] Nature; and, therefore, however complicated it may appear, it is, in fact, the moſt ſimple; be⯑cauſe, as I formerly remarked, whatever moſt frequently happens, is, in itſelf, however it may ſeems to us, the moſt ſimple.

Beſides, the notion of the Ariſtotelians, that females have no ſeminal fluid, cannot receive our aſſent, if we conſider the ſtrong reſemblance of children to their mothers, and that mules, mu⯑lattoes, and mongrels of every kind, uniformly reſemble the mother more than the father; and, if it be farther conſidered, that the generating organs of the female, like thoſe of the male, are properly formed for preparing and receiving a ſeminal fluid, we ſhall be eaſily induced to be⯑lieve the exiſtence of ſuch a fluid, whether it re⯑ſides in the ſpermatic veſſels, the teſticles, or the ovaria, or proceeds, by irritation, from the la⯑cunae of De Graaf, which are ſituated at the neck and near the orifice of the uterus.

But we muſt examine Ariſtotle's ideas more fully, as, of all the antients, this great philoſopher has treated the ſubject of generation in the moſt extenſive manner. He diſtinguiſhes animals into three claſſes: 1. Thoſe that have blood, and, with few exceptions, propagate by copula⯑tion; 2. Thoſe that have no blood, and, being hermaphrodites, produce of themſelves without copulation; and, 3. Thoſe that proceed from putrefaction, and have no parents of any kind. I ſhall firſt remark, that this diviſion is exceed⯑ingly [75] improper: Though it be true, that ani⯑mals having blood are diſtinguiſhed into male and female, it is by no means equally true, that bloodleſs animals are, for the moſt part, herma⯑phrodites: For the only hermaphrodites we know, are land-ſnails and worms; but we are uncertain whether all ſhell-animals, and all thoſe which have no blood, be alſo hermaphrodites. This muſt be learned from the particular hiſtories of theſe animals. And, with regard to thoſe that are alledged to proceed from putrefaction, as Ariſ⯑totle gives no enumeration of them, many ob⯑jections occur; for moſt ſpecies which the an⯑tients believed to proceed from putrefaction, have, by the moderns, been diſcovered to pro⯑ceed from eggs.

Ariſtotle makes a ſecond diviſion of animals, namely, into thoſe who have the faculty of pro⯑greſſive motion, and thoſe who have no ſuch fa⯑culty. All animals who move, and have blood, are diſtinguiſhed by ſexes: But thoſe which, like oyſters, adhere to one place, or hardly move at all, have no ſexes, and, in this reſpect, re⯑ſemble plants; and it is only, he obſerves, from difference in bulk that they have been diſtin⯑guiſhed into male and female. It muſt be ac⯑knowledged, that we are ſtill uncertain whether ſhell-animals have ſexes: Among oyſters, ſome individuals are fertile, and others not. The fer⯑tile individuals are diſtinguiſhed by a delicate edging or border which ſurround their bodies, [76] and they are called males^*. Our obſervations on this ſubject are extremely limited.

But to proceed. The male, according to A⯑riſtotle, contains the principle of motion, and the female the material part of generation. The organs deſtined for this purpoſe are different in different animals. Of theſe the teſticles are the chief in males, and the uterus in females. Qua⯑drupeds, birds, and cetaceous animals, have teſ⯑ticles; fiſhes and ſerpents are deprived of them; but they have two canals for the reception and maturation of the ſemen: Theſe parts, ſo eſſential to generation, are always double both in males and females; and, in the male, they retard the motion of that part of the blood which goes to the formation of ſemen. This he proves from the example of birds whoſe teſticles ſwell conſi⯑derably during the ſeaſon of their amours, but afterwards diminiſh ſo greatly that they can hardly be diſcovered.

All quadrupeds covered with hair, and the ce⯑taceous fiſhes, as whales and dolphins, are vivi⯑parous: But vipers and cartilagious animals are not properly viviparous; becauſe they produce an egg within their own bodies, previous to the excluſion of the live animal. Oviparous ani⯑mals are of two kinds; thoſe which produce perfect eggs, as birds, lizards, turtles, &c. and thoſe which produce imperfect eggs, as fiſhes, whoſe eggs augment and come to perfec⯑tion [77] after they have been depoſited in the water by the female: And, in every ſpecies of ovipa⯑rous animals, except birds, the females are larger than the males, as in fiſhes, lizards, &c.

After remarking theſe general varieties in the animal kingdom, Ariſtotle begins with exami⯑ning the opinion of the antient philoſophers, that the ſemen, both of the male and the female, was extracted from all parts of the body; and he diſſents from this opinion; becauſe, ſays he, though children often reſemble both father and mother, they ſometimes alſo reſemble their grand⯑fathers. Beſides, they reſemble their fathers and mothers in the voice, in the hair, in the nails, and in the gate and manner of walking. Now, he proceeds, it is impoſſible for the ſemen to come from the hair, from the voice, from the nails, or from any external quality, as that of the mode of walking. Infants, therefore, re⯑ſemble not their parents, becauſe the ſemen proceeds from all parts of the body, but for o⯑ther reaſons. I will not expoſe the weakneſs of theſe arguments; but ſhall only remark, that this great man appears to have been anxious to differ from the ſentiments of former philoſophers: And I am perſuaded, that, whoever peruſes his trea⯑tiſe on generation, will diſcover, that a ſtrong paſſion for eſtabliſhing a ſyſtem different from that of the antients, obliges him uniformly to prefer arguments of little probability, to the [78] force of proofs, when they ſtand in oppoſition to the general principles of his philoſophy.

The ſeminal liquor of the male, according to Ariſtotle, is ſecreted from the blood; and the menſtrual fluid of the female is likewiſe a ſecre⯑tion from the blood, and the only matter which contributes to generation. Females, he conti⯑nues, have no other prolific fluid; no mixture, therefore, of male and female fluid takes place: This he attempts to prove by obſerving, that ſome women conceive without pleaſure; that few emit any fluid during the time of copula⯑tion; that, in general, thoſe who are brown, and have a maſculine air, have no emiſſion; and yet their powers of procreation are not leſs than thoſe of a fairer complection and more delicate appearance, who emit copiouſly. Thus, he concludes, women furniſh nothing for the pur⯑poſes of generation, but the menſtrual blood. This blood is the matter of generation, and the male fluid contributes nothing but the form: The male fluid is the efficient cauſe, and the prin⯑ciple of motion; it is to generation what the ſculptor is to a block of marble: The ſeminal fluid is the ſculptor, the menſtrual blood the marble, and the foetus the figure. The men⯑ſtrual blood receives from the male ſemen a kind of ſoul, which gives it life and motion. This ſoul is neither material nor immaterial, becauſe it can neither act upon matter, nor augment the menſtrual blood, which is the only matter neceſ⯑ſary [79] to generation. It is a ſpirit, ſays our phi⯑loſopher, ſimilar to that of the element of the ſtars. The heart is the firſt production of this ſoul, which is the cauſe of its own growth, and of the growth and diſpoſition of all the other members. The menſtrual blood contains the capacities of all the parts of the foetus; the ſoul or ſpirit of the male ſemen makes the heart be⯑gin to act, and communicates to it the powers of beſtowing action on the other viſcera; and, in this manner, the different parts of the animal are ſucceſſively unfolded. All this appears clear and luminous to our philoſopher. He has only one doubt, namely, whether the blood or the heart is firſt realized. And of this he doubted not with⯑out reaſon; for, though he adopted the opinion that the heart received its exiſtence firſt, Harvey has ſince alledged, from arguments ſimilar to thoſe of Ariſtotle, that the blood, and not the heart, is firſt realized.

Thus have I given a ſhort view of what A⯑riſtotle has delivered on the ſubject of genera⯑tion, and ſhall leave the reader to conſider whe⯑ther any ſyſtem of the antients be more obſcure, or more abſurd, than that which he has endea⯑voured to eſtabliſh. His ſyſtem, however, has been adopted by moſt men of learning. Har⯑vey has borrowed many of Ariſtotle's notions; but he has alſo adopted ſome of his own, which are by no means better founded. It is not ſur⯑priſing that Ariſtotle's theory of generation, [80] which was a reſult of his ſyſtem of philoſophy, where form and matter are the great principles, where vegetable and ſenſitive ſouls are the a⯑gents of nature, and where final cauſes are real objects, ſhould have been received in the ſchools: But it is not a little aſtoniſhing to ſee a phyſi⯑cian and an acute obſerver, like Harvey, carried down the ſtream, while, at the ſame time, moſt philoſophers followed the ſentiments of Hippo⯑crates and Galen, which we ſhall afterwards take notice of.

We mean not to convey a diſadvantageous idea of Ariſtotle by the account we have given of his theory of generation. We might, with equal propriety, judge of Deſcartes by his trea⯑tiſe on man. What theſe two philoſophers have remarked concerning the formation of the foetus ſhould rather be conſidered in the light of detached obſervations, or as conſequences which each of them drew from their principles of philoſophy, than as compleat ſyſtems. Ariſ⯑totle admits, with Plato, final and efficient cauſes: The latter are the ſenſitive and vegetable ſouls, that give form to matter, which, in itſelf, is only a capacity of receiving forms: And as, in ge⯑neration, the female furniſhes the greateſt quan⯑tity of matter, and as it was repugnant to his ſyſtem of final cauſes, that any effect ſhould be produced by two cauſes, when one was ſufficient for the purpoſe, he concludes, that the woman alone contains the matter neceſſary for procrea⯑tion: [81] Again, another of his principles was, that matter, in itſelf, has no form, and that form is a being diſtinct from matter; he therefore main⯑tains, that the male furniſhes the form, and, of courſe, that he contributes nothing material.

Deſcartes, on the contrary, admitted into his philoſophy only a few mechanical principles. By theſe he attempted to explain the formation of the foetus; and he imagined, that he under⯑ſtood, and was able to communicate to others, the manner in which a living organized body could be formed by the laws of motion alone. The principles he employed were different from thoſe of Ariſtotle. But both of them, inſtead of directing their inquiries to the thing itſelf, in place of examining it with impartiality, only conſidered it in relation to their philoſophic prin⯑ciples, which could never be applied with ſuc⯑ceſs to the nature of generation, becauſe it de⯑pends, as has been already ſhown, upon very different principles. Deſcartes, however, admits the exiſtence and neceſſary concurrence of the ſeminal fluids of both ſexes. He allows that both furniſh ſomething material for the purpoſes of generation; and that the fermentation occaſion⯑ed by a mixture of the two fluids, is the cauſe of the formation of the foetus.

Hippocrates, who lived about five or ſix hun⯑dred years before Ariſtotle, taught an opinion, which was adopted by Galen, and by moſt phy⯑ſicians, for many ages. He maintained the ex⯑iſtence [82] of a female fluid; and even that both male and female had two fluids, the one ſtrong and active, the other weaker and more inactive^*. A concurrence of the two ſtronger fluids produ⯑ced a male child, and, of the two weaker, a fe⯑male. Thus, according to Hippocrates, there exiſt two kinds of ſeminal fluids both in the male and in the female. This notion he ſupports in the following manner: Several women, who produced only girls by their firſt huſband, have had boys by their ſecond; and the ſame thing has often happened to men who have had two wives. Suppoſing this to be fact, it admits of an eaſy explanation, without having recourſe to two different fluids peculiar to each ſex; for the women who had girls only by the firſt huſband, and boys by the ſecond, furniſhed a greater quantity of particles proper for generation du⯑ring the firſt, than the ſecond marriage; or the ſecond huſband furniſhed a greater quantity of generating particles during the time of the ſe⯑cond marriage, than the firſt. If, at the mo⯑ment of conception, the organic particles of the male are more abundant than thoſe of the fe⯑male, a male child is the reſult; and, when the organic particles of the female moſt abound, a female child is the conſequence: It is not, there⯑fore, ſurpriſing, that the huſband ſhould be foil⯑ed with ſome women, and have the ſuperiority over others.

[83] It is farther alledged by Hippocrates, that the male ſemen is ſecreted from the ſtrongeſt and moſt eſſential fluids of the body; and he thus explains the manner in which the ſecretion is performed: 'Venae et nervi,' ſays he, 'ab omni corpore in pudendum vergunt, quibus dum ali⯑quantulum teruntur, et caleſcunt ac implentur, velut pruritus incidit, ex hoc toti corpori volup⯑tas ac caliditas accidit; quum vero pudendum teritur et homo movetur, humidum in corpore caleſcit ac diſſunditur, et a motu conquaſſatur ac ſpumeſcit, quemadmodum alii humores omnes conquaſſati ſpumeſcunt.'

'Sic autem in homine ab humido ſpumeſcente id quod robuſtiſſimum eſt ac pinguiſſimum ſecer⯑nitur, et ad medullam ſpinalem venit; tendunt enim in hanc ex omni corpore viae, et diſſun⯑dunt ex cerebro in lumbos ac in totum corpus et in medullam: Et ex ipſa medulla procedunt viae, ut et ad ipſam humidum perferatur et ex ipſa ſecedat; poſtquam autem ad hanc medul⯑lam genitura pervenerit, procedit ad renes, ac enim via tendit per venas; et ſi renes fuerint exulcerati, aliquando etiam ſanguis defertur: A renibus autem tranſit per medios teſtes in pu⯑dendum, procedit autem non qua urina, verum alia ipſi via eſt illi contigua^*,' &c.

It will, doubtleſs, be perceived by anatomiſts, that Hippocrates errs in tracing the route of the ſeminal fluid. But this affects not his hypotheſis, [84] that the ſemen proceeds from every part of the body, and particularly from the head; becauſe, he remarks, thoſe who have had the veins be⯑hind their ears cut, ſecrete only a weak and often an unfertile ſemen. The female likewiſe ſheds a ſeminal fluid ſometimes within the uterus, and ſometimes without, when the orifice is too open. The male ſemen enters the uterus and mixes with that of the female; and, as each has two ſpecies of fluid, the one ſtrong and the other weak, if both of them furniſh the ſtrong kind, a male foetus is the conſequence; and, if both furniſh only the weak kind, the reſult is a fe⯑male: Beſides, if in the mixture there are more particles of the male than of the female fluid, the child will reſemble the father more than the mother; et c contra. Here we might aſk him, what would happen, when the fluid of the one was ſtrong, and that of the other weak? I can⯑not conceive what reply could be made to this queſtion; and, therefore, we are warranted to reject the opinion of two diſtinct fluids in each ſex as perfectly chimerical.

Let us now attend to his account of the for⯑mation of the foetus. The ſeminal fluids firſt mix in the uterus, and gradually thicken by the heat of the mother. The mixture extracts the ſpirit of heat, and when too warm, part of the heat eſcapes into the air. But a cold ſpirit is likewiſe conveyed to it by the reſpiration of the mother: Thus a cold and a hot ſpirit alternately [85] enter the mixture, give life to it, and cover its ſurface with a pellicle, which aſſumes a round figure, becauſe the ſpirits acting in the center, expand the matter equally on all ſides. I have ſeen, ſays this great phyſician, a foetus of ſix days old: It was a ball of liquor incloſed in a pellicle. The liquor was reddiſh; and the pellicle was interſperſed with red and colourleſs veſſels. In the middle of it there was a ſmall e⯑minence, which I apprehended to be the umbi⯑lical veſſels, by which the foetus receives nou⯑riſhment and the ſpirit of reſpiration from the mother. A ſecond covering or pellicle gradu⯑ally forms above the firſt. Abundance of nou⯑riſhment is furniſhed by the menſtrual blood, which coagulates by degrees, and is converted into fleſh. This fleſh gradually articulates as it grows; and the ſpirit beſtows this form upon it. Every part proceeds to take its proper place; the ſolid particles unite together; the moiſt particles aſſociate by themſelves; every thing ſearches for what is analogous to it; and, in fine, the foetus, by theſe cauſes and means, is completely formed.

This ſyſtem is more rational, and leſs obſcure than that of Ariſtotle; becauſe Hippocrates en⯑deavours to explain every particular appearance, and borrows only one general principle from the philoſophy of his times, namely, that heat and cold produce ſpirits, and that theſe ſpirits have the power of arranging and of beſtowing [86] figure upon matter. He treats his ſubject more like a phyſician than a philoſopher; but Ariſtotle explains the phaenomena of generation more as a metaphyſician than a naturaliſt. It is for this reaſon that the errors of Hippocrates are parti⯑cular and leſs apparent, and that thoſe of Ariſ⯑totle are general and evident.

Theſe two great men have each had their fol⯑lowers. Almoſt all the philoſophers of the ſchools adopted Ariſtotle's theory of generation, while moſt phyſicians adhered to the theory of Hippocrates; and, in this manner, 17 or 18 cen⯑turies paſſed without the appearance of any thing new upon this myſterious ſubject.

At laſt, upon the revival of literature, ſome a⯑natomiſts began to inveſtigate the nature of gene⯑ration; and Fabricius ab Aquapendente was the firſt who thought of making a courſe of experi⯑ments upon the impregnation and expanſion of the eggs of fowls, the ſubſtance of which we ſhall lay before the reader.

He diſtinguiſhes the matrix of a hen into two parts, the one ſuperior, and the other infe⯑rior. The ſuperior part, which he calls the ovarium, is an aſſemblage of a great number of ſmall yellow eggs, of a round figure, the ſizes of which vary from that of a muſtard ſeed to that of a walnut. Theſe eggs are attached to one another by foot-ſtalks, and the whole ſome⯑what reſembles a bunch of grapes. The ſmalleſt [87] eggs are white, and they turn yellower in pro⯑portion as they increaſe.

Having examined thoſe yellow eggs imme⯑diately after a communication with the male, he could perceive no ſenſible difference; he ſaw none of the male ſemen in any part of the eggs: He therefore concluded, that the whole eggs, and even the ovarium itſelf, were rendered fer⯑tile by a ſubtile ſpirit which iſſues from the male ſemen; and, he adds, that, in order to prevent the eſcape of this fecundating ſpirit, nature has placed, at the external orifice of the vagina of birds, a membranous valve which permits the ſeminal ſpirit to enter freely into the vagina, but prevents its return.

When an egg is detached from the common pedicle, it gradually deſcends, through a wind⯑ing canal, into the inferior part of the matrix. This canal is filled with a liquor very ſimilar to the white of an egg. It is here that the egg receives its white liquor, the membrane in which it is incloſed, the two cords (chalazae) that run through the white, and join it to the yolk, and the ſhell which is ſuddenly formed immediately before excluſion. Theſe cords, according to our author, are the part of the egg which is im⯑pregnated by the ſeminal ſpirit of the male; and it is here alſo that the rudiments of the foetus firſt appear. The egg is not only the true ma⯑trix, or the place where the chick is formed, but the whole buſineſs of generation depends upon [88] it. The egg is the great agent in generation; it furniſhes both the matter and the organs. The ſubſtance of the cords is the matter of which the chick is formed; the white and the yok afford it nouriſhment; and the ſeminal ſpi⯑rit of the male is the efficient cauſe. This ſpirit communicates to the cords, firſt, an alterant quality, then a forming quality, and, laſtly, a power of augmenting, &c.

Theſe obſervations of Fabricius, it is apparent, lead not to any clear idea of generation. At the ſame time that this anatomiſt was making his experiments, which was about the middle of the ſixteenth century, the famous Aldrovandus^* made ſome remarks upon eggs. But, as Har⯑vey properly obſerves of him, he followed more the authority of Ariſtotle than of experiment. The deſcription he gives of the chick in the egg is by no means exact. Volcher Coiter, one of his pupils, ſucceeded better than his maſter; this writer, together with Pariſanus, a Venetian phy⯑ſician, have each given deſcriptions of the chick in the egg, which Harvey prefers to all the o⯑thers.

This celebrated anatomiſt, who firſt diſcover⯑ed the circulation of the blood, has given an ex⯑cellent treatiſe on generation. He flouriſhed about the middle of laſt century, and was phy⯑ſician to Charles I. of England. As he was o⯑bliged to follow this unhappy Prince during his [89] misfortunes, he loſt, among other papers, what he had written concerning the generation of in⯑ſects; and it appears, that he compoſed from memory his treatiſe on the generation of birds and of quadrupeds. I ſhall give a ſhort view of his remarks, of his experiments, and of his the⯑ory.

Harvey alledges, that men, and all other ani⯑mals, proceed from eggs; that, in viviparous a⯑nimals, the firſt produce of conception is a kind of egg; and that the only difference between the viviparous and oviparous is, that, in the for⯑mer, the foetuſes begin to exiſt, increaſe, and acquire their full growth in the uterus; but that, in the oviparous animals, the foetuſes begin to exiſt in the body of the mother, where they are in the form of eggs; and it is only after their excluſion that they become real foetuſes. And it deſerves to be remarked, ſays he, that, in ovi⯑parous animals, ſome retain their eggs till they be perfect, as birds, ſerpents, and oviparous qua⯑drupeds; and that others exclude their eggs be⯑fore they are perfect, as fiſhes, cruſtaceous, and teſtaceous animals. The eggs laid by thoſe crea⯑tures are only the rudiments of eggs, which af⯑terwards acquire membranes and a white, and attract nouriſhment from the matter with which they are ſurrounded. There are even, he adds, inſects, caterpillars, for example, which are only imperfect eggs; they ſearch for their nouriſh⯑ment, and, at the end of a certain time, they ar⯑rive [90] at the ſtate of a chryſalis, which is a perfect egg. Another difference may ſtill be remarked in oviparous animals: The eggs of hens, and other birds, are of all different ſizes; but thoſe of fiſhes, frogs, &c. who lay them before they are perfect, are all of the ſame ſize. He indeed obſerves, that, in pigeons, who lay two eggs, all the ſmall eggs that remain in the ovarium are of the ſame bulk; and that the two only which are next to be excluded exceed the ſize of the reſt. The ſame thing happens in cartilaginous fiſhes, as in the ray, which only brings to maturity two eggs at a time, all the reſt being of different ſizes, like thoſe of the hen.

He next deſcribes anatomically the parts ne⯑ceſſary to generation; and remarks, that the ſi⯑tuation of the anus and vulva in birds differ from thoſe of all other animals, the anus being placed before, and the vulva behind^*. And, with regard to the cock, and all ſmall birds, he al⯑ledges, that they have no proper penis, and that they generate by rubbing, without any intro⯑miſſion. But male-ducks, geeſe, and oſtriches, are amply provided with this inſtrument.

Hens produce eggs without the intervention of the cock; but, though perfect, they are fewer in number, and unfertile. He credits not the common opinion, that a few days inter⯑courſe with the cock are ſufficient to impregnate all the eggs which a hen will lay during the year; [91] but he acknowledges, that he ſeparated a hen from the cock for 20 days, and that all the eggs ſhe laid were fecundated. As long as the egg remains attached to the ovarium, it is nouriſhed by the veſſels of the common pedicle; but, when it ſeparates from this pedicle, it receives the white liquor and the ſhell from the matter with which the canal of the uterus is filled.

The two cords (chalazae) which Aquapen⯑dente conſidered to be the germ, or part produ⯑ced by the male ſemen, are found in unimpreg⯑nated, as well as impregnated eggs; and Harvey properly obſerves, that theſe parts neither pro⯑ceed from the male, nor receive the impregna⯑tion. The part of the egg which receives the impregnation is a ſmall white circle ſituated up⯑on the membrane that covers the yolk, and has the appearance of a cicatrice about the ſize of a lentil. Harvey likewiſe remarks, that this ci⯑catrice is found in all eggs, whether they be fe⯑cundated or not; and that thoſe are deceived who imagine it to be produced by the ſeed of the male. It is of the ſame ſize and form in freſh eggs as in thoſe which have been long kept. But, as ſoon as the proceſs of hatching is begun, whether by means of artificial heat, or by the heat of the hen, this ſmall mark or cicatrice gradual⯑ly augments and dilates like the pupil of the eye. This is the firſt change, and it is viſible after a few hours incubation.

[92] When the egg has been heated for 24 hours, the yolk, which was formerly in the centre, riſes [...] the cavity at the thick end of the egg This cavity continues to enlarge by the [...] of the more fluid part of the white; and the heavieſt part of the white falls down to the ſmall end. The cicatrice or ſpeck on the membrane of the yolk, is elevated along with it, and applies itſelf to the membrane which lines the cavity at the thick end. This ſpeck is now as large as a pea; and a white point is diſtinguiſhable in the middle of it, with ſeveral circles, of which this point appears to be the common centre.

At the end of the ſecond day, theſe circles are larger and more conſpicuous, and they divide the ſpeck ſometimes into two, and ſometimes into three parts, of different colours. A ſmall external pro⯑tuberance likewiſe appears, which nearly reſem⯑bles a little eye, with a white point or cataract on the pupil. Between the circles a liquor, as tranſparent as cryſtal, is contained by means of a very thin membrane. The ſpeck, which is now become a kind of bubble, or liquid globe, appears as if it were ſituated in the white, rather than on the membrane of the yolk. On the third day the tranſparent liquor, as well as the mem⯑brane in which it is incloſed, is conſiderably aug⯑mented. On the 4th, a ſmall line of blood, of a purple colour, appears on the circumference of the bubble; and, at a little diſtance from the [93] centre, we perceive a dot or point, of a bloody colour, which beats like a heart. It is viſible at every diaſtole, and diſappears during the ſyſtole. From this animated point two ſmall blood-veſſels iſſue, and terminate in the membrane which con⯑tains the tranſparent cryſtalline liquor. Theſe blood-veſſels ſet off from the ſame place, nearly in the ſame manner as the roots of a tree ſet off from the trunk; and it is in the angle which theſe roots form with the trunk, and in the mid⯑dle of the liquor, that the animated point is ſi⯑tuated.

Towards the end of the fourth, or beginning of the fifth day, the animated point is ſo much enlarged, that it has the appearance of a ſmall bladder filled with blood; and, by its contractions and dilatations, it is alternately filled and emp⯑tied. On the ſame day we diſtinctly perceive, that this bladder is divided into two parts, each of which dilates and contracts in the ſame manner. Round the ſhorteſt of the blood-veſſels deſcribed above, a kind of cloud appears, which, though tranſparent, obſcures the view of the veſſel. E⯑very hour this cloud becomes thicker; it attach⯑es itſelf to the root of the blood-veſſel, and ſeems to depend from it like a ſmall globe. This globe extends and appears to divide into three parts, one of which is globular, and larger than the o⯑ther two; and here we perceive the rudiments of two eyes, and of the whole head: And, at the end of the fifth day, we ſee, in the remain⯑der of this lengthened globe, the beginnings of the vertebrae.

[94] On the ſixth day, the parts of the head are more apparent. We diſtinguiſh the coats of the eyes, the thighs, and wings; and then the liver, the lungs, and the beak. The foetus now begins to move and to ſtretch out its head, though no⯑thing but the viſcera are yet formed; for the thorax, the abdomen, and all the external co⯑verings of the fore part of the body, are ſtill wanting. At the end of this day, or the begin⯑ning of the ſeventh, the claws begin to be vi⯑ſible; the chick opens and moves its beak; and the anterior parts of the body begin to cover the viſcera. On the ſeventh day, the chick is en⯑tirely formed; and, from this time till it iſſues from the egg, nothing happens but an expanſion of all the parts it acquired during the firſt ſeven days. The feathers appear on the 14th or 15th day; and, on the 21ſt, it eſcapes from the egg, by breaking the ſhell with its bill.

Theſe experiments of Harvey appear to have been made with the greateſt exactneſs and fide⯑lity. In the ſequel, however, we ſhall demon⯑ſtrate their imperfection, and that the author has probably fallen into the common error of ma⯑king experiments, with a view to eſtabliſh his fa⯑vourite hypotheſis, that the firſt animated point which appeared was the heart. But, before pro⯑ceeding to this object, it is proper to give an ac⯑count of his other experiments.

Every body knows the many experiments of Harvey made upon female deer. They receive [95] the male about the middle of September. A few days after copulation, the horns ^* of the ute⯑rus appear to be thicker and more fleſhy than uſual: They are, at the ſame time, more lax and flabby; and, in each of their cavities, five ca⯑runculae, or ſoft warts, appear. About the 26th or 28th of September, the uterus is ſtill thicker; the five carunculae are ſwelled nearly to the ſize and form of a nurſe's nipple. On opening them with a ſcalpel, they appeared to be filled with an infinite number of white points. Harvey pre⯑tends to have remarked, that, neither now, nor immediately after copulation, had the ovarium ſuffered any change; and that he never could diſcover, after repeated trials, the leaſt drop of male ſemen in the uterus.

Towards the end of October, or the beginning of November, when the females were ſeparated from the males, the thickneſs of the horns began to diminiſh; the internal ſurfaces of their cavi⯑ties were ſwelled, and ſeemed to be glued toge⯑ther. The carunculae ſtill remained; and the whole reſembled the ſubſtance of the brain, be⯑ing ſo ſoft that it could not be touched. Har⯑vey tells us, that on the 13th or 14th of Novem⯑ber, he perceived filaments, like thoſe of a ſpi⯑der's web, which traverſed the cavities of the horns, and even that of the uterus itſelf. Theſe filaments aroſe from the ſuperior angle of the [96] horns, and, by their number, formed a kind of membrane or empty coat. A day or two after⯑wards, this coat or ſac was filled with a white, aqueous, viſcid matter, and adhered to the ute⯑rus by means of a ſpecies of mucilage; and the adheſion was moſt ſenſible at the ſuperior part of the uterus, where the rudiments of the placenta began then to appear. In the third month, this ſac contained an embryo of two fingers breadth in length, and alſo an internal ſac, called the amnios, incloſing a tranſparent cryſtalline liquor, in which the foetus ſwam. The foetus, at firſt, was only an animated point, like what ap⯑peared in the hen's egg. Every thing now pro⯑ceeded and terminated in the ſame manner as deſcribed with regard to the chick, with this on⯑ly difference, that the eyes of the chick appear⯑ed much ſooner than thoſe of the deer. The a⯑nimated point was viſible about the 19th or 20th of November. A day or two afterwards, the oblong body, which contained the rudiments of the foetus, made its appearance. In ſix or ſeven days more, the foetus was ſo compleatly formed, that all its members, and even its ſex, were di⯑ſtinguiſhable. But the heart and viſcera were ſtill bare; and it was not till a day or two after, that they were covered with the abdomen and thorax. This is the laſt work, the ſlating of the edifice.

From theſe experiments upon hens and deer, Harvey concludes, that all female animals have [97] eggs; that in theſe eggs a ſeparation of a tranſpa⯑rent cryſtalline liquor, contained in a ſac (amnios), takes place, and that another external ſac (chorion) incloſes the whole liquors of the egg; that the firſt thing which appears in the cryſtalline li⯑quor is an animated ſanguineous point; and, fi⯑nally, that the formation of viviparous animals is effected in the ſame manner with that of the oviparous: The following is the account which he gives of the generation of both.

Generation, he obſerves, is an operation of the uterus alone; for not a drop of the male ſemen ever enters it. The uterus conceives by a kind of contagion, communicated to it by the ſemen of the male, nearly in the ſame manner as the load-ſtone communicates a magnetic virtue to iron. This male contagion acts not only on the uterus, but on the whole body of the female, which is entirely fecundated, though the uterus alone poſſeſſes the faculty of conception, in the ſame manner as the brain has the ſole power of conceiving ideas. The ideas conceived by the brain are ſimilar to the images of the objects tranſmitted to it by the ſenſes; and the foetus, which may be regarded as the idea of the uterus, is ſimilar to that which produces it. This is the reaſon why children reſemble their fathers, &c.

I will follow the ſyſtem of our anatomiſt no farther: What has been ſaid is ſufficient to en⯑able the reader to judge of it. But we have re⯑marks of importance to make concerning his [98] experiments. He has repreſented them in a manner the moſt plauſible and inſinuating. He appears to have often repeated them, and to have taken every neceſſary precaution to avoid fallacy and deception; and, of courſe, we are led to think that he has ſeen every thing which poſſi⯑bly could be diſcovered. Uncertainty and ob⯑ſcurity, however, are perceptible in his deſcrip⯑tions. His obſervations are related from memo⯑ry; and he ſeems, though he often maintains the contrary, to have made Ariſtotle, more than experience, his guide; for he has ſeen every thing in eggs, and very little more, than was mentioned by that philoſopher. That the moſt material of his obſervations were made long be⯑fore his own time, we ſhall be convinced by at⯑tending to what follows.

Ariſtotle knew, that the cords (chalazae) in eggs were of no uſe in the generation of the chick: 'Quae ad principium lutei grandines haerent, nil conferunt ad generationem, ut qui⯑dam ſuſpicantur^*.' Pariſanus, Volcher Coiter, Aquapendente, &c. had remarked the ſmall ci⯑catrice, as well as Harvey. Aquapendente be⯑lieved it to be of no uſe; but Pariſanus main⯑tained that it was formed by the male ſemen, or, at leaſt, that the white point in the middle of the cicatrice was the ſemen of the male, and that it was the rudiments of the foetus, 'Eſtque,' ſays he, 'illud galli ſemen alba et tenuiſſima tu⯑nica [99] abductum, quod ſubſtat duabus communi⯑bus toti ovo membranis,' &c. Hence the only diſcovery proper to Harvey is his having re⯑marked the exiſtence of this cicatrice, both in fecundated and unfecundated eggs; for the o⯑ther writers had obſerved, as well as he, the di⯑latation of the circles, and the growth of the white point. Theſe are all the remarks Harvey has made in his account of the two firſt days of incubation; what he mentions concerning the third day is only a repetition of what Ariſtotle delivers in the ſixth book and fourth chapter of his hiſtory of animals: 'Per id tempus aſcendit jam vitellus ad ſuperiorem partem ovi acutio⯑rem, ubi et principium ovi eſt et foetus exclu⯑ditur; corque ipſum apparet in albumine ſan⯑guinei puncti, quod punctum ſalit et movet ſe⯑ſe inſtar quaſi animatum; ab eo meatus vena⯑rum ſpecie duo, ſanguine pleni, flexuoſi, qui, creſcente foetu, feruntur in utramque tunicam ambientem, ac membrana ſanguineas fibras ha⯑bens eo tempore albumen continet ſub meati⯑bus illis venarum ſimilibus; ac paulo poſt diſ⯑cernitur corpus puſillum initio, omnino et can⯑didum, capite conſpicuo, atque in eo oculis maxime turgidis qui diu ſic permanent, ſero e⯑nim parvi ſiunt ac conſidunt. In parte autem corporis inferiore nullum extat membrum per initia, quod reſpondeat ſuperioribus. Meatus autem illi qui a corde prodeunt, alter ad cir⯑cumdantem [100] cumdantem membranam tendit, alter ad lute⯑um, officio umbilici.'

Harvey, becauſe Ariſtotle ſays that the yolk riſes to the ſmall end of the egg, concludes that he had ſeen nothing himſelf, but had received his information from ſome other pretty accurate obſerver. In this accuſation, Harvey evidently injures Ariſtotle; for the riſing of the yolk to either end, ſolely depends upon its poſition du⯑ring the time of incubation; for the yolk, being lighter than the white, uniformly mounts to the top, whether the large or the ſmall end of the egg be uppermoſt. This obſervation we owe to William Langley, a phyſician in Dordrecht, who made experiments on the hatching of eggs in the year 1655, about 20 years before Harvey's time^*.

But, to return to the paſſage we have quoted. It is apparent, that the cryſtalline liquor, the animated point, the two circles, the two blood-veſſels, &c. are deſcribed by Ariſtotle in the ſame manner as they were ſeen by Harvey. This anatomiſt maintains, that the animated point is the heart, that the heart is the firſt part of the foetus which is formed, and that the viſcera and other members ſucceed. All this has been men⯑tioned by Ariſtotle, and ſeen by Harvey; and yet it is by no means conſonant to truth. To be aſſured of this fact, we have only to repeat the ſame experiments, or to read with attention thoſe [101] of Malpighius^*, which were made about 50 years after the trials of Harvey.

Malpighius carefully examined the cicatrice, which is the eſſential part of the egg; he found that it was large in impregnated eggs, and ſmall in thoſe which had received no impregnation; and he diſcovered, that, in eggs which had ne⯑ver been ſit upon, the white point, mentioned by Harvey as the firſt part that becomes animated, is a ſmall purſe or bubble ſwimming in the li⯑quor bounded by the firſt circle; and that the embryo is viſible in the centre of this purſe. The membrane of the purſe, which is the am⯑nios, being exceedingly thin and tranſparent, allowed him to ſee diſtinctly the foetus within it. Malpighius, from this firſt obſervation, con⯑cludes with propriety, that the foetus exiſts in the egg before incubation, and that the rudi⯑ments of the embryo are even then deeply rooted. It is unneceſſary to mention how much this ex⯑periment differs from the opinion of Harvey; for he had obſerved nothing begun to be formed during the two firſt days of incubation; and, in his eſtimation, the firſt veſtige of a foetus is the animated point, which appears not till the third day. But Malpighius diſcovered that the rudi⯑ments of the whole foetus exiſt before incuba⯑tion is commenced.

After aſcertaining this important fact, Mal⯑pighius proceeded to examine the cicatrice of unimpregnated eggs, which, as formerly re⯑marked, [102] is ſmaller than in thoſe that have received an impregnation. Its margin is often irregular, and its texture ſometimes differs in different eggs. Near its centre, in place of a bubble including the foetus, there is a globular mole or unorgani⯑zed maſs, which, when opened, preſents nothing like regularity or arrangement of parts: It has only ſome appendages filled with a thick but tranſparent liquor; and this unformed maſs is ſurrounded and enveloped in ſeveral concentric circles.

After ſix hours incubation, the cicatrice is con⯑ſiderably enlarged; and, in its centre, a bubble or globule formed by the amnios is eaſily di⯑ſtinguiſhable. This globule is filled with a fluid, in the middle of which the head and back-bone of the chick viſibly appear. Six hours after this, every thing is enlarged, and, of courſe, more apparent to the eye. In ſix hours more, that is, 18 hours after the commencement of in⯑cubation, the head is larger, and the ſpine is lengthened; and, at the end of 24 hours, the head of the chick appears in a bended poſture, and the ſpine is of a whitiſh colour. The ver⯑tebrae are ranged on each ſide of the ſpine, like ſmall globules; and, nearly at the ſame time, the wings begin to ſprout, and the head, neck, and breaſt, are lengthened. At the end of 30 hours, nothing new appears; but all the parts are enlarged, and eſpecially the amnios. Round this membrane, may be remarked the umbilical [103] veſſels, which are of a dark colour. In 38 hours, the chick has acquired more ſtrength; its head is very large, and three veſicles appear in it ſur⯑rounded with membranes, which likewiſe in⯑clude the ſpine of the back, through which, however, the vertebrae are ſtill viſible. At the end of 40 hours, it was admirable to obſerve, continues our author, the chick living in the centre of the liquor of the amnios. The back-bone was increaſed, the head was bended, the veſicles of the brain were leſs bare, the rudi⯑ments of the eyes appeared, the heart beat, and the blood circulated. Here Malpighius deſcribes veſſels and the circulation of the blood; and he thought, with reaſon, that, though the heart did not beat till 38 or 40 hours after incubation was begun, it nevertheleſs exiſted before, as well as the other parts of the chick. But, on exami⯑ning the heart in a dark chamber, he obſerved nothing like luminous ſparks iſſuing from it, as Harvey ſeems to inſinuate.

At the end of the ſecond day, the foetus ap⯑peared ſwimming in the liquor of the amnios; the head, which ſeemed to be compoſed of ve⯑ſicles, was bended; the back-bone and verte⯑brae were lengthened; the heart, which hung out of the breaſt, beat three times ſucceſſively, becauſe the fluid it contains is puſhed from the auricle into the ventricles, from the ventricles into the arteries, and, laſtly, into the umbilical veſſels. He remarks, that, having ſeparated the [104] chick from the white of the egg, the motion of the heart continued for a whole day. In 14 hours more, or 62 hours from the beginning of the incubation, the chick, though ſtronger, re⯑mained ſtill with its head bended in the liquor of the amnios: Veins and arteries were perceived among the veſſels of the brain; and the linea⯑ments of the eyes, and of the ſpinal marrow, ap⯑peared. At the end of three days, the body of the chick was crooked. Beſide the two eyes, five veſicles filled with liquor appeared in the head; the rudiments of the thighs and of the wings were diſcernible; the body began to take on fleſh; and the pupils of the eyes, and likewiſe the cryſtalline and vitreous humours, were diſ⯑tinguiſhable. At the termination of the fourth day, the veſicles of the brain were nearer each other; the proceſſes of the vertebrae were long⯑er; the wings and the thighs had become ſtrong⯑er, in proportion as they grew longer; the whole body was covered with an unctuous fleſh; the umbilical veſſels had pierced through the abdo⯑men; and the heart was concealed within the breaſt, which was now ſhut up by a thin mem⯑brane. On the fifth, and at the end of the ſixth day, the veſicles of the brain began to be covered; the ſpinal marrow, which was now more ſolid, was divided into two parts, and advanced along the trunk; the thighs and wings were longer, and the wings were unfolded; the abdomen was ſhut and tumified; the liver was diſtinctly viſible, [105] and of a dark colour; the two ventricles of the heart beat; the body of the chick was covered with ſkin; and the points of the feathers began to appear. On the ſeventh day, the head was very large; the brain was covered with its mem⯑branes; the beak appeared between the two eyes; the wings, the thighs, and the legs, had acquired their perfect form; the heart ſeemed to be compoſed of two ventricles, like two con⯑tiguous globules, united at their ſuperior part with the auricles; and two ſucceſſive pulſes were remarked both in the ventricles and auricles, as if there had been two ſeparate hearts.

But I will follow Malpighius no farther. The remainder of the detail regards the growth and perfection of the parts till the chick breaks the ſhell in which it is incloſed, and becomes an in⯑habitant of a new world. The heart is the laſt part that aſſumes its proper figure, by the union of its ventricles, which happens not till the e⯑leventh day.

We are now in a condition to form a diſtinct judgment concerning the value of Harvey's ex⯑periments. It is probable that this celebrated anatomiſt did not make uſe of the microſcope, (which was, indeed, very imperfectly known in his days), otherwiſe he never would have affirm⯑ed, that there was no difference between the cicatrice of impregnated and unimpregnated eggs; he never would have ſaid, that the ſemen of the male produced no change upon the egg, [106] and particularly upon the cicatrice; he never would have advanced, that nothing was percep⯑tible before the end of the third day; that the animated point appeared firſt; and that the white point was transformed into the animated point: He would have perceived that the white point was the bubble or globule which contained the whole apparatus of generation; and that all the rudiments of the foetus commenced there from the moment of receiving the impregnation of the cock: He would likewiſe have diſcovered, that, without this impregnation, it contains no⯑thing but an unformed maſs, which could never become animated; becauſe, in fact, it is not organized like an animal, and becauſe it is only after this maſs, which ought to be regarded as a collection of the organic particles of the female ſemen, is penetrated by the organic particles of the male ſemen, that an animal is formed. This formation is inſtantaneous; but the motions of the new animal are imperceptible till 40 hours after the proceſs of incubation has commenced: He would not have aſſured us, that the heart is firſt formed, and that the other parts are ſucceſ⯑ſively joined to it by juxta-poſition; ſince it is apparent, from the experiments of Malpighius, that the rudiments of all the parts are formed at once, but that they become perceptible only in proportion as they are ſucceſſively unfolded: Laſtly, if he had ſeen, as Malpighius ſaw, he would not have poſitively aſſerted, that no im⯑preſſion [107] of the male ſeed remained in the eggs, and that it was only by contagion that they were impregnated, &c.

It is alſo proper to remark, that what Harvey has ſaid concerning the parts of generation of the cock is by no means exact. He affirms, that the cock has no penis capable of entering the vagina of the hen. It is certain, however, that this animal, in place of one penis, has a couple, which both act at the ſame time; and this action is a vigorous compreſſion, if not an actual copulation^*. It is by this double organ that the cock throws his ſeminal liquor into the uterus of the hen.

Let us now compare Harvey's experiments upon female deer with thoſe of de Graaf upon female rabbits; and, though de Graaf believed, as Harvey did, that all animals proceed from eggs, we ſhall find a very great difference in the manner in which theſe two anatomiſts have per⯑ceived the firſt formation, or rather the expan⯑ſion, of the foetuſes of viviparous animals.

After exerting every effort to prove, by ar⯑guments drawn from comparative anatomy, that the teſticles of viviparous females are true ova⯑ria, Graaf explains the manner in which the eggs are detached from the ovaria, and fall into the horns of the uterus; and then he relates the remarks he made upon a rabbit which he diſ⯑ſected half an hour after copulation. The horns [108] of the uterus, he ſays, were uncommonly red; there was no change either in the ovaria, or in the eggs which they contained; and there was not the leaſt appearance of ſemen in the vagina, in the uterus, or in the Fallopian tubes.

Having diſſected another rabbit, ſix hours af⯑ter copulation, he obſerved, that the follicles, or coats, which, in his eſtimation, contain the eggs in the ovarium, were become red; but he found no male ſemen either in the ovaria or any where elſe. Twenty hours after copulation, he diſſect⯑ed a third; he remarked in one ovarium three, and in the other five follicles much altered; for inſtead of being clear and limpid, they were become opaque and reddiſh. In another, diſ⯑ſected twenty-ſeven hours after copulation, the horns of the uterus, and the ſuperior canals which terminate in them, were ſtill more red, and their extremities embraced the ovarium on all ſides. In another, which was opened forty hours after copulation, he found in one ovarium ſeven, and in the other three follicles changed. Fifty-two hours after copulation, he examined another, and found in one ovarium four changed follicles, and one in the other; and having open⯑ed theſe follicles, he diſcovered in them a kind of glandular liquor, with a ſmall cavity in the middle, where he could perceive no fluid, which made him ſuſpect that the tranſparent liquor uſually contained in the follicles, and which, he ſays, is incloſed in its own membranes, might [109] have been diſcharged by ſome kind of rupture. He ſearched for this matter in the canals which terminate in the horns of the uterus, and in the horns themſelves; but he found nothing. He only remarked, that the membranes which line the horns of the uterus were much ſwelled. In another rabbit, diſſected three days after copu⯑lation, he obſerved, that the ſuperior extremity of the canal, which terminates in the horns of the uterus, ſtraitly embraced the ovarium on every ſide: And, having ſeparated it from the ovarium, he remarked, in the right ovarium, three follicles ſomewhat larger and harder than uſual. After ſearching with great care the ca⯑nals above mentioned, he diſcovered, he ſays, an egg in the right canal, and two more in the right horn of the uterus, ſo ſmall that they ex⯑ceeded not muſtard ſeeds. Theſe little eggs had each two membranes, and the internal one was filled with a very limpid liquor. Having examined the other ovarium, he found four changed follicles; three of them were whiter, and had likewiſe ſome limpid liquor in their centres; but the fourth was of a darker colour, and contained no liquor, which made him ſuſ⯑pect that the egg had eſcaped from it. He there⯑fore ſearched the correſponding canal and horn of the uterus; he found an egg in the ſuperior extremity of the horn, which was exactly ſimi⯑lar to thoſe he had diſcovered in the right horn. He alledges, that the eggs, when they are ſepa⯑rated [110] from the ovarium, are ten times leſs than before their ſeparation; and this difference in ſize, he imagines, is owing to the eggs, while in the ovarium, containing another matter, namely, the glandulous liquor which he remarked in the follicles.

Four days after copulation, he opened another rabbit, and he found in one ovarium four, and in the other three follicles void of eggs: In the horns correſponding to the ovaria, he found four eggs on one ſide, and three in the other. Theſe eggs were larger than thoſe he had diſcovered three days after copulation. They were nearly of the ſize of the lead-ſhot uſed for ſhooting ſmall birds; and he remarked, that, in theſe eggs, the interior membrane was ſeparated from the exterior, and appeared as if a ſecond egg was contained within the firſt. In another, diſ⯑ſected five days after copulation, he found five empty follicles in the ovaria, and an equal num⯑ber of eggs in the uterus, to which they adhe⯑red very firmly. Theſe eggs were as large as the ſhot employed for killing hares, and the in⯑ternal membrane was ſtill more apparent than in the laſt experiment. Having opened another rabbit, ſix days after copulation, he found in one of the ovaria ſix empty follicles, but only five eggs in the correſponding horn of the ute⯑rus, and they ſeemed to be all accumulated into one maſs: In the other ovarium, he ſaw four empty follicles, and found but one egg in the [111] correſponding horn. Theſe eggs were of the ſize of the largeſt fowling ſhot. Seven days af⯑ter copulation, our anatomiſt opened another rabbit, and he found in the ovaria ſome empty follicles, which were larger, harder, and more red than thoſe he had formerly obſerved; and he perceived as many tranſparent tumors in dif⯑ferent parts of the uterus; and, having opened them, he took out the eggs, which were as large as ſmall piſtol bullets. The internal membrane was more diſtinct than formerly; and within this membrane he ſaw nothing but a very clear li⯑quor. In another, diſſected eight days after co⯑pulation, he found in the uterus the tumors or cells which contain the egg; but they adhered ſo ſtrongly to the uterus, that he could not de⯑tach them. In another, which he opened nine days after copulation, he found the cells con⯑taining the eggs greatly enlarged, and he per⯑ceived in the middle of the liquor incloſed by the internal membrane a ſmall thin cloud. In another, which he opened ten days after copu⯑lation, the ſmall cloud was thicker and darker, and formed an oblong body like a little worm. Laſtly, twelve days after copulation, he diſtinctly perceived the embryo, which, though two days before, it was only an oblong body, was now ſo apparent, that he could diſtinguiſh its different members. In the region of the breaſt, he ſaw two red and two white points, and, in the ab⯑domen, a mucilaginous reddiſh ſubſtance. Four⯑teen [112] days after copulation, the head of the foe⯑tus was large and tranſparent; the eyes were prominent; the mouth was open; the rudi⯑ments of the ears appeared; the back-bone was whitiſh, and bended towards the ſternum, and ſmall blood-veſſels aroſe from each ſide of it, the ramifications of which extended along the back as far as the legs: The two red points were conſiderably enlarged, and appeared like the rudiments of the ventricles of the heart; on each ſide of the red points he ſaw two white ones, which were the rudiments of the lungs. In the abdomen he ſaw the rudiments of the li⯑ver, which was reddiſh, and a ſmall body twiſted like a thread, which was the ſtomach and inte⯑ſtines. After this, till the 31ſt day, when the female rabbit brings forth, there was nothing to be remarked but the gradual expanſion and growth of the parts which were already formed.

From theſe experiments, De Graaf concludes, that all viviparous females have eggs; that theſe eggs are contained in the ovaria or teſticles; that they cannot be ſeparated till they are fecun⯑dated by the ſemen of the male; becauſe, ſays he, the glandular liquor, by means of which the eggs are enabled to eſcape from their follicles, is not ſecreted till after an impregnation by the male. He alledges, that thoſe who imagine they have ſeen pretty large eggs in three days, have been deceived; becauſe, in his opinion, the eggs, though fecundated, remain longer in the ovarium, and, [113] in place of augmenting, they become ten times leſs than formerly, and they never begin to grow till after their deſcent from the ovaria in⯑to the uterus.

By comparing theſe obſervations of Graaf with thoſe of Harvey, we will eaſily perceive that the latter has miſſed the principal facts: And, though there are ſeveral errors both in the reaſoning and in the experiments of De Graaf, this anatomiſt, as well as Malpighius, have diſ⯑covered themſelves to be better obſervers than Harvey. They agree in all fundamental points, and both of them contradict Harvey. He per⯑ceived not the alterations which take place in the ovaria; he ſaw not in the uterus thoſe ſmall globules which contain the materials of genera⯑tion, and which are called eggs by De Graaf; he never ſuſpected that the foetus exiſted in this egg; and, though his experiments give us tole⯑rably exact ideas concerning what happens du⯑ring the growth of the foetus, he furniſhes no information concerning the commencement of fecundation, nor concerning the firſt expanſion of the foetus. Schrader, a Dutch phyſician, who had a great veneration for Harvey, acknow⯑ledges that he cannot be truſted in many articles, and particularly in what relates to the firſt for⯑mation of the embryo; for the chick really ex⯑iſts in the egg before incubation; and, he ſays, that Joſeph of Aromatarius was the firſt who [114] made this material obſervation^*. Beſides, though Harvey alledged that all animals proceeded from eggs, he never imagined that the teſticles of fe⯑males contained eggs; and it was only from a compariſon between the ſac, which he believed to be formed in the uterus of viviparous ani⯑mals, with the growth and covering of the eggs in oviparous animals, that he maintained that all animals were produced from eggs; and even this is only a repetition of what Ariſtotle had ſaid before him. Steno was the firſt who pre⯑tended to have diſcovered eggs in the ovaria of females. He ſays, that, in diſſecting a female ſea-dog, he perceived eggs in the teſticles, though this animal be viviparous; and he adds, that the teſticles of women are analogous to the ovaria of oviparous animals, whether the eggs them⯑ſelves fall into the uterus, or only the matter which they contain. Steno was the firſt who diſcovered theſe ſuppoſed eggs; De Graaf is willing to aſſume the diſcovery to himſelf; and Swammerdam warmly diſputes the point with him, and alledges that Van-Horn had ſeen them before De Graaf. This laſt writer, it is true, has been accuſed of aſſerting many things which have been contradicted by experiments: He even pretended, that a certain judgment might be formed of the number of foetuſes in the ute⯑rus, by the number of cicatrices or empty fol⯑licles in the ovaria. In this he is contradicted [115] by the experiments of Verrheyen^*, by thoſe of M. Mery^†, and by ſome of his own, where he found fewer eggs in the uterus, than cicatrices in the ovaria. Beſides, we ſhall demonſtrate that what he ſays concerning the ſeparation of the eggs, and the manner in which they deſcend into the uterus, is by no means exact; that no eggs exiſt in the teſticles of females; that what is ſeen in the uterus is not an egg; and that the ſyſtems which have been deduced from the ob⯑ſervations of this celebrated anatomiſt are per⯑fectly chimerical.

This pretended diſcovery of eggs in the teſti⯑cles of females attracted the attention of moſt anatomiſts. They only found, however, in the teſticles of viviparous females, ſmall bladders; thoſe they heſitated not to conſider as real eggs, and, therefore, they called the teſticles ovaria, and the veſicles eggs. They aſſerted alſo, like De Graaf, that theſe eggs differed in ſize in the ſame ovarium; that the largeſt in the ovaria of women exceeded not the bulk of a ſmall pea; that they are very ſmall in young girls; but that they increaſed with age and intercourſe with men; that not above 20 could be reckoned in each ovarium; that theſe eggs are fecundated in the ovarium by the ſpirituous part of the male ſemen; that they then ſeparate and fall into the uterus by the Fallopian tubes, where the [116] foetus is formed of the internal ſubſtance of the egg, and the placenta of its external part; that the glandulous matter, which exiſts not in the ovarium till after a fruitful embrace, compreſſes the egg, and excludes it from the ovarium, &c. But, though Malpighius, who examined mat⯑ters more accurately, detected many errors com⯑mitted by thoſe anatomiſts even before they were received; yet moſt phyſicians adopted the o⯑pinion of De Graaf, without regarding the ob⯑ſervations of Malpighius, which were neverthe⯑leſs of the greateſt importance, and which re⯑ceived much weight from the experiments of his diſciple Valiſnieri.

Malpighius and Valiſnieri, of all naturaliſts, appear to have written with moſt judgment and acuteneſs on the ſubject of generation. We ſhall, therefore, give an account of their experi⯑ments and remarks.

Malpighius, having examined the teſticles of a number of cows and other female animals, aſſures us, that he found, in the teſticles of all of them, veſicles of different ſizes, whether the females were very young or adults. Theſe ve⯑ſicles are enveloped in a pretty thick membrane, the inſide of which is interſperſed with blood-veſſels; and they are filled with a kind of lymph or liquor, which coagulates and hardens by the heat of a fire, like the white of an egg.

In proceſs of time, a firm yellow body ad⯑heres to the teſticles. It is prominent, and in⯑creaſes [117] to the ſize of a cherry, and occupies the greateſt part of the ovarium. This body con⯑ſiſts of ſeveral angular lobes, the poſition of which is very irregular, and it is covered with a coat or membrane interſperſed with nerves and blood-veſſels. The form and appearance of this yellow body varies conſiderably at different times. When it exceeds not the ſize of a grain of millet, it is roundiſh, and its ſubſtance, when cut, has a warty appearance. We often find an external covering round the veſicles of the o⯑varia, which conſiſts of the ſame ſubſtance with the yellow bodies.

When the yellow body has become nearly of the ſize of a pea, it reſembles a pear; and, in the centre of it, there is a ſmall cavity filled with liquor. The ſame thing may be remarked when it is as large as a cherry. In ſome of theſe yellow bodies, after they have arrived at full maturity, Malpighius affirms that he ſaw, to⯑wards the centre, a ſmall egg with its appen⯑dages, about the ſize of a millet ſeed; and, after they had diſcharged theſe eggs, they were flaccid and empty. They then reſembled a cavernous canal; and the void cavities were as large as peas. He conceived that Nature deſigned this yellow glandular body for the preſervation of the egg, and for making it eſcape from the te⯑ſticles; and that, perhaps, it contributed to the formation of the egg; conſequently, he remarks, the veſicles which are at all times found in the [118] ovarium, and always differ in ſize, are not the true eggs which receive the impregnation, but only ſerve to produce the yellow bodies in which the eggs are formed. Beſides, though theſe yel⯑low bodies are not always found in every ova⯑rium; yet we always find the rudiments of them. Malpighius found the marks of them in new born heifers, in cows with calf, and in preg⯑nant women; and, therefore, he properly con⯑cludes, that theſe yellow glandular bodies are not, as De Graaf aſſerts, an effect of impregna⯑tion. The yellow bodies, he remarks, produce unfecundated eggs, which fall out of the ova⯑rium independent of any communication with the male, and alſo thoſe which fall after impreg⯑nation. When the impregnated eggs fall into the uterus, every thing proceeds in the manner deſcribed by De Graaf.

Theſe obſervations of Malpighius demonſtrate, that the teſticles of females are not real ovaria; that the veſicles they contain are not eggs; that theſe veſicles never fall into the uterus; and that the teſticles, like thoſe of males, are only reſer⯑voirs containing a liquor which may be regarded as female ſemen in an imperfect ſtate. This ſe⯑men is matured in the yellow glandular bodies, of which it fills the internal cavities, and flows out after the yellow bodies have acquired their full ſize.

But, before we form a judgment concerning this important point, we muſt attend to the re⯑marks of Valiſnieri.

[119] In the year 1692, Valiſnieri began his expe⯑riments upon the teſticles of the ſow. The te⯑ſticles of the ſow differ from thoſe of cows, of mares, of ſheep, of ſhe-aſſes, of female-dogs, of ſhe-goats, of women, and of moſt viviparous animals; for they reſemble a ſmall bunch of raiſins, the grains of which are round and pro⯑minent on the outſide; between theſe grains are ſmaller ones, not yet arrived at maturity. Theſe grains appear not to be covered with a common membrane. They are, ſays he, analogous to the yellow bodies obſerved in cows by Malpi⯑ghius; they are round, and of a reddiſh colour; their ſurface is interſperſed with blood-veſſels, like the eggs of viviparous animals; and the whole grains together form a maſs that is larger than the ovarium. With a little addreſs, theſe grains may be ſeparated from the ovarium, and each of them, after ſeparation, leaves a nitch or depreſſion.

Theſe glandular bodies are not of the ſame colour in every ſow. In ſome they are more red; in others more clear; and they are of all ſizes, from the ſmalleſt ſeed, to that of a raiſin. On opening them, a triangular cavity appears, filed with a limpid liquor, which coagulates with heat, and becomes white, like that which is contained in the veſicles. Valiſnieri expected to find the egg in ſome of theſe cavities: But in this he was diſappointed; though he made a careful ſearch into all the glandular bodies of a [120] number of ſows, and other animals, he could never diſcover the egg, which Malpighius af⯑firms he found once or twice.

Under theſe glandular bodies, the veſicles of the ovarium appeared. They were more or leſs numerous, according as the glandular bodies were larger or ſmaller; for, in proportion to the largeneſs of the glandular bodies, the veſicles diminiſhed. Some veſicles were of the ſize of a lentil, and others exceeded not that of a millet ſeed. In the teſticles, when raw, from 20 to 35 veſicles might be reckoned; but, when boiled, a much greater number appear, and they are ſo firmly attached, that they cannot be ſeparated without breaking ſome of them.

Having examined the teſticles of a young ſow, which had never brought forth, he found, as in the others, the glandular bodies; and their tri⯑angular cavities were likewiſe filled with lymph; but he could not diſcover any eggs either in the one or the other. The veſicles of this young ſow were more numerous than in thoſe which had brought forth, or thoſe which were impregnated at the time of examination. In the teſticles of another ſow, which was far advanced in preg⯑nancy, Valiſnieri found two of the largeſt glan⯑dular bodies, which were flaccid and empty, and others, of a leſſer ſize, in their ordinary ſtate; and, in ſeveral others which he diſſected when with young, he remarked, that the number of glandular bodies was always greater than the [121] number of foetuſes. This confirms what we obſerved concerning the experiments of De Graaf, and proves that they are by no means exact. What he calls follicles of the ovarium are only the glandular bodies, the number of which al⯑ways exceeds that of the foetuſes. In the ova⯑ria of a ſow, two or three months old, the teſ⯑ticles were pretty large, and interſperſed with veſicles of a conſiderable ſize. Among the ve⯑ſicles, the beginnings of four glandular bodies appeared in one teſticle, and of ſeven in the other.

After theſe experiments upon ſows, Valiſnieri repeats thoſe of Malpighius upon cows, and he found them to be exactly conformable to truth. He indeed acknowledges, that he was never able to diſcover the egg which Malpighius ima⯑gined he had ſeen once or twice in the interior cavity of the glandular bodies. After a fruitleſs ſearch in the teſticles of ſo many different fe⯑males, it was natural to think, that Valiſnieri would at leaſt have doubted the exiſtence of ſuch eggs. But prejudice in favour of ſyſtem made him admit, contrary to his own experience, the exiſtence of eggs, which neither he nor any other man ever ſaw, or will ſee.

It is, perhaps, impoſſible to make a greater number, or more exact experiments, than Valiſ⯑nieri has done. Among other animals, he ex⯑amined the ewe, and found, that ſhe has never any more glandular bodies in her teſticles than [122] foetuſes in the uterus. In young ewes, which were never impregnated, there is but one glan⯑dular body in each teſticle, and, when one is emptied, it is ſucceeded by another; if a ewe has one foetus in the uterus, ſhe has only one glandular body in her teſticles; and if ſhe has two foetuſes, ſhe has likewiſe two glandular bo⯑dies. This glandular body occupies the greateſt part of the teſticle; and, after it is emptied and diſappears, another begins to grow for the pur⯑poſe of a future generation.

In the teſticles of a ſhe-aſs, he found veſicles as large as ſmall cherries, which is an evident proof that they are not eggs, as it would be im⯑poſſible for them to paſs, by the Fallopian tubes, into the uterus.

The teſticles of female wolves, dogs, and foxes, are covered with a membrane, like a purſe, which is an expanſion of that which ſurrounds the horns of the uterus. In a bitch which began to be in ſeaſon, but had not been approached by the male, Valiſnieri found the internal part of this purſe, which does not adhere to the teſticle, moiſtened with a liquor that reſembled whey, and two glandular bodies in the right teſticle, about two lines in diameter, and which occu⯑pied nearly the whole extent of the teſticle. Each glandular body had a ſmall nipple, with a diſtinct fiſſure, from which, without preſſing it, there iſſued a liquor like clear whey; he there⯑fore concluded, that this liquor was the ſame [123] which he found in the purſe. He blew into this fiſſure with a pipe, and the whole glandular body immediatly ſwelled; and, having introdu⯑ced a briſtle, he eaſily penetrated to the bottom of it. He opened the body on that ſide where he had introduced the briſtle, and found an in⯑ternal cavity which communicated with the nipple, and contained a conſiderable quantity of liquor. Valiſnieri was always in hopes of diſ⯑covering the egg; but theſe hopes, notwithſtand⯑ing all his reſearches, were uniformly fruſtrated. He likewiſe found, in the left teſticle, two glan⯑dular bodies very ſimilar to thoſe in the right. He boiled two of theſe glandular bodies, hoping, that, by this means, he might diſcover the egg, but ſtill without any meaſure of ſucceſs.

Having diſſected another bitch four or five days after ſhe had received the male, he found in the teſticles three glandular bodies exactly ſimilar to the former. He ſearched every where for the egg; but he was ſtill diſappointed. By the aſſiſtance of the microſcope, he diſcovered the glandular bodies to be a net-work compoſed of an infinite number of globular veſicles, which ſerved to filtre the liquor which iſſued through the nipple.

He then opend another bitch which was not in ſeaſon, and having tried to introduce air be⯑tween the teſticle and the purſe which covered it, he found that it dilated like a bladder filled with air. Having removed the purſe, he diſco⯑vered [124] two glandular bodies upon the teſticles; but they had neither nipple nor fiſſure, and no liquor diſtilled from them.

In another bitch that had brought forth about five whelps two months before, he found five glandular bodies; but they were much diminiſh⯑ed in ſize, and they began to diſappear without leaving any cicatrices; there remained only a ſmall cavity in their centre; but it contained no liquor.

Not ſatisfied with theſe and many other ex⯑periments, Valiſnieri, who paſſionately deſired to diſcover this pretended egg, called together the beſt anatomiſts his country afforded, and, a⯑mong others, M. Morgagni; and, having open⯑ed a young bitch that was for the firſt time in ſeaſon, and that had been covered three days be⯑fore, they examined the veſicles of the teſticles, the glandular bodies with their nipples, their ca⯑nals, and the liquor in their internal cavities; but they could perceive no eggs. He then, with the ſame intention, made experiments on ſhe-goats, foxes, cats, a number of mice, &c. In the teſticles of all theſe animals, he uniformly found the veſicles, and frequently the glandular bodies with the liquor they contained; but no egg ever appeared.

In fine, being deſirous of examining the te⯑ſticles of women, he had an opportunity of o⯑pening a young country-woman, who had been ſome years married, and who was killed by a [125] fall from a tree. Though of a robuſt and vigo⯑rous conſtitution, ſhe had never born any chil⯑dren. He endeavoured to diſcover if the cauſe of her barrenneſs exiſted in the teſticles; and he found that the veſicles were all filled with a blackiſh and corrupted matter.

In a young girl of eighteen years of age, who had been brought up in a convent, and who had every appearance of real virginity, he found the right teſticle a little longer than the left: It was of an oval figure, and its ſurface was ſomewhat unequal. This inequality was occaſioned by five or ſix veſicles which protruded on the outſide of the teſticle. One of theſe veſicles, which was more prominent than the reſt, he opened, and a quan⯑tity of lymph ruſhed out of it. This veſicle was ſurrounded with a glandular ſubſtance, in the ſhape of a creſcent, and of a reddiſh yellow co⯑lour. He cut the teſticle tranſverſely, and found a number of veſicles filled with limpid liquor; and he remarked, that the Fallopian tube of this teſticle was redder and ſomewhat longer than the other, as he had often obſerved in other animals when they were in ſeaſon.

The left teſticle was whiter, and its furface more ſmooth; for, though ſome veſicles were a little prominent, none of them were in the form of nipples; they were all ſimilar to each other, and the correſponding Fallopian tube was nei⯑ther ſwelled nor red.

[126] In the teſticles of a girl, aged five years, he found the teſticles with their veſicles, their blood-veſſels, and their nerves.

In the teſticles of a woman of ſixty years, he diſcovered ſome veſicles, and the veſtiges of a glandular ſubſtance, like large points of an ob⯑ſcure yellowiſh brown colour.

From all theſe obſervations, Valiſnieri con⯑cludes, that the work of generation is carried on in the female teſticles, which he continued to re⯑gard as ovaria, though he never could find any eggs in them, and though, on the contrary, he had diſcovered that the veſicles were not eggs. He ſays, likewiſe, that, for the impregnation of the egg, it is not neceſſary that the male ſemen ſhould enter the uterus. He ſuppoſes, that the egg eſcapes through the nipple of the glandular body, after being impregnated in the ovarium; that it then falls into the Fallopian tube; that it gradually deſcends, and at laſt attaches itſelf to the uterus: He adds, that he is fully perſuaded, that the egg is concealed in the cavity of the glandular body, though neither he nor any other anatomiſt was ever able to diſcover it.

In his eſtimation, the ſpirit of the male ſeed aſcends into the ovarium, penetrates the egg, and gives motion to the foetus which previouſly exiſted in it. In the ovarium of the original mother of mankind, he obſerves, were eggs, which contained not only all the children ſhe produced, but of the whole human race. If [127] this chain of infinite individuals contained in one, be incomprehenſible to us, it is entirely owing to the imbecillity of our minds, of which we have daily proofs. But it is not, therefore, leſs conſonant to truth, that all the animals which have exiſted, or can exiſt, were created at once, and were all included in their firſt mothers. The reſemblance of children to their parents is owing, he continues, to the imagination, which acts ſo forcibly on the foetus as to produce ſtains, mon⯑ſtroſities, diſorder of parts, and extraordinary concretions, as well as perfect ſimilarities.

This ſyſtem of eggs, though it explains no⯑thing, and has no foundation in Nature, would have obtained the univerſal ſuffrages of phyſi⯑cians, if, nearly about the ſame time, another opinion had not ſprung up, founded upon the diſcovery of ſpermatic animals.

This diſcovery, which we owe to Leeuwen⯑hoek and Hartſoeker, was confirmed by Andri, Valiſnieri, Bourguet, and many other obſervers. I ſhall relate what has been advanced concern⯑ing thoſe ſpermatic animals which are found in the ſemen of all males. Their number is ſo great, that the ſemen ſeems to be entirely com⯑poſed of them; and Leeuwenhoek pretends to have ſeen many millions of them in a drop leſs than the ſmalleſt grain of ſand. Though none of them appear in females, they are found in the emitted ſemen of all males, in the teſticles, and in the veſiculae ſeminales. When the ſemen of [128] a man is expoſed to a moderate heat, it thickens, and the motion of all the animalcules is ſud⯑denly ſtopped. But, when allowed to cool, it dilutes, and the animals continue in motion till the liquor again thickens by evaporating. The more this fluid is diluted, the number of animalcules are augmented; and, when greatly diluted by the addition of water to it, the whole ſubſtance of the fluid ſeems to be compoſed of animals When the motion of the animalcules is about to ceaſe, either on account of heat or of drying, they appear to approach nearer each other, to have a common circular motion in the centre of the ſmall drop under obſervation, and to periſh, all of them, at the ſame inſtant. But, when the quantity of liquor is greater, it is eaſy to diſtinguiſh them dying in ſucceſſion.

Theſe animalcules are ſaid to be of different figures in different animals; but they are all long, thin, without any members, and move with rapidity in every direction. The fluid in which they are contained, as formerly remark⯑ed, is much heavier than blood. The ſemen of a bull, when chemically analyzed by Verrheyen, yielded firſt phlegm, then a conſiderable quan⯑tity of foetid oil, a very ſmall proportion of vo⯑latile ſalt, and more earth than he expected^*. This author was ſurpriſed that he could draw no ſpirit from the diſtillation of this liquor; and, as he imagined it contained a great quantity of [129] ſpirits, he attributed the evaporation of them to their ſubtility. But may we not ſuppoſe, with more probability, that it contains little or no ſpirits? Neither the conſiſtence, nor the odor of this fluid, indicate the preſence of ardent ſpirits, which never abound but in fermented liquors; and, with regard to volatile ſpirits, the horns, bones, and ſolid parts of animals, afford more of them than the fluids. What has received the appellation of ſeminal ſpirits, aura ſeminalis, a⯑mong anatomiſts, has, perhaps, no exiſtence; and it is certain, that the moving bodies appa⯑rent in the ſeminal fluid, are not agitated by theſe ſpirits. But, that we may be enabled to pro⯑nounce more clearly concerning the nature of the ſemen, and of its animalcules, we ſhall pre⯑ſent the reader with the principal obſervations which have been made on the ſubject.

Leeuwenhoek, having examined the ſemen of a cock, perceived a number of animals ſimilar to river eels; but they were ſo minute, that 50,000 of them were not equal in bulk to a grain of ſand. Of thoſe in the ſemen of a rat, it re⯑quired, he ſays, many millions to make the thickneſs of a hair, &c. This excellent obſer⯑ver was perſuaded, that the whole ſubſtance of the ſemen was only a maſs of animalcules. He ſaw theſe animalcules in the ſemen of men, of quadrupeds, of birds, of fiſhes, and of inſects. In the ſemen of a graſs-hopper, the animalcules were long, and extremely thin. They appeared, [130] he ſays, to be attached by their ſuperior end; and the other end, which he calls their tail, had a briſk motion, like that of the tail of a ſerpent when its head is fixed. In the ſemen of young animals, when examined before they have any ſexual appetite, he alledges that he ſaw the ſame minute animals, and that they had no motion: But, when the ſeaſon of love arrived, the ani⯑malcules moved with great vivacity.

In the ſemen of a male frog, he ſaw animal⯑cules; but, at firſt, they were imperfect, and had no motion: Some time after, he found them alive. They were ſo minute, he obſerves, that ten thouſand of them were only equal in bulk to a ſingle egg of the female.

In the ſemen of a man and that of a dog, he pretended to ſee two ſpecies of animalcules, re⯑ſembling males and females. Having ſhut up the ſemen of the dog in a ſmall vial, he ſays, that a great number of animalcules died the firſt day; that, on the ſecond and third day, ſtill more of them died; and that few of them were alive on the fourth day. But, having repeated this experiment on the ſemen of the ſame dog, he found, at the end of ſeven days, the animal⯑cules as briſk and lively as if they had been newly extracted from the animal: And, having opened a bitch that, ſome time before the expe⯑riment, had been three times covered by the ſame dog he could not perceive, with his naked eye, any male ſemen in the uterus or its appendages; [131] but, by the aſſiſtance of the microſcope, he found the ſpermatic animals of the dog in both horns of the uterus: In that part of the uterus which is neareſt the vagina, he diſcovered great num⯑bers, which evidently proves, ſays he, that the male ſemen enters the uterus, or, at leaſt, that the ſpermatic animals of the dog had arrived their by their own motion, which enables them to paſs over 4 or 5 inches in half an hour. In the uterus of a female rabbit, which had juſt re⯑ceived the male, he obſerved an infinite number of ſpermatic animals. He remarks, that the bo⯑dies of theſe animals are round; that they have long tails; and that they often change their fi⯑gure, eſpecially when the fluid in which they ſwim begins to dry up.

Theſe experiments of Leeuwenhoek were re⯑peated by ſeveral people, who found them ex⯑actly conſonant to truth. But Dalenpatius, and ſome others, who were inclined to exceed Leeu⯑wenhoek in acuteneſs of viſion, alledged that, in the ſemen of a man, they not only found a⯑nimals reſembling tadpoles, whoſe bodies ap⯑peared to be as large as a grain of corn, with tails about four times as long as their trunks, and who moved with great agility; but, what is ſtill more amazing, Dalenpatius ſaw one of theſe animals break through its coat or covering: It was then no more an animalcule, but a real human body, in which he eaſily diſtinguiſhed [132] the two arms and legs, the breaſt and the head^*. But it is apparent, from the very figures, given by this author, of the embryo which he pretend⯑ed to have ſeen eſcape from its covering, that the fact is abſolutely falſe. He believed that he ſaw what he deſcribes; but he was deceived; for this embryo, according to his deſcription, was more completely formed, at the time of its tranſ⯑migration from the condition of a ſpermatic worm, than it is in the uterus of the mother at the end of the fourth or fifth week. Hence this obſervation of Dalenpatius, inſtead of being confirmed by future experiments, has been re⯑jected by all naturaliſts, the moſt acute of whom have only been able to diſcover in the ſeminal fluid of man, round or oblong bodies, which ap⯑pear to have long tails, but no members of any kind.

One would be tempted to think that Plato had been acquainted with thoſe ſpermatic animals which are transformed into men; for, at the end of his Timacus^†, he ſays, 'Vulva quoque matrixque in foeminis eadem ratione animal avidum generandi, quando procul a foetu per aetatis fiorem, aut ultra diutius detinetur, aegre fert moram ac plurimum indignatur, paſſimque per corpus oberrans, meatus ſpiritus intercludit, reſpirare non ſinit, extremis vexat anguſtiis, morbis denique omnibus premit, quouſque u⯑trorumque [133] cupido amorque quaſi ex arboribus foetum fructumve producunt, ipſum deinde de⯑cerpunt, et in matricem velut agrum inſpar⯑gunt: Hinc animalia primum talia, ut nec prop⯑ter parvitatem videantur, necdum appareant formata, concipiunt; mox quae conflaverant, explicant, ingentia intus enutriunt, demum e⯑ducunt in lucem, animaliumque generationem perficiunt.' Hippocrates, in his treatiſe De Diaeta, ſeems likewiſe to inſinuate that the ſe⯑men of animals is full of animalcules. Demo⯑critus talks of certain worms which aſſume the human figure; and Ariſtotle tells us, that the firſt men iſſued from the earth in the form of worms. But neither the authority of Plato, of Ariſtotle, of Hippocrates, of Democritus, nor of Dalenpatius, will ever be able to beſtow credi⯑bility on a notion which is repugnant to the re⯑peated experience and obſervation of all thoſe who have hitherto made inquiries into this ſub⯑ject.

Valiſnieri and Bourguet perceived ſmall worms in the ſemen of a rabbit: One of their extremities was longer than the other; they were very active in their motions, and beat the fluid with their tails: Sometimes they raiſed them⯑ſelves to the top of the liquor, and ſometimes ſunk to the bottom; at other times they turned round, and twiſted like ſerpents: In fine, ſays Valiſnieri, I clearly perceived them to be real animals: 'E gli riconobbi, e gli giudicai ſenza [134] dubitamento alcuno per veri, veriſſimi, arcive⯑riſſimi vermi^*.' This author, though preju⯑diced in favour of the ovular ſyſtem, admitted the actual exiſtence of ſpermatic animals.

M. Andry pretends, that he could find no a⯑nimals in human ſemen previous to the age of puberty; that they exiſt not in the ſemen of very old men; that there are few of them in thoſe who are affected with the venerial diſeaſe, and that theſe few are in a languiſhing ſtate; that none of them appear alive in impotent per⯑ſons; and that the animalcules in the ſemen of men have a larger head than thoſe of other a⯑nimals, which correſponds, he obſerves, with the figure of the foetus and infant; and he adds, that thoſe who uſe women too frequently have generally few or no animalcules in their ſemen.

Leeuwenhoek, Andry, and others, exerted e⯑very effort againſt the egg-ſyſtem: They diſco⯑vered in the ſemen of all males living animal⯑cules; they proved that theſe animalcules could not be regarded ſimply as inhabitants of this fluid, ſince the quantity of them was larger than that of the fluid itſelf; and ſince nothing ſimi⯑lar to them exiſted either in the blood, or in any other of the animal fluids: They maintained, that, as females furniſhed no animalcules, their fecundity was ſolely derived from the males; that the exiſtence of living animals in the ſe⯑men throw more light upon the nature of gene⯑ration [135] than all the former diſcoveries on this ſubject; becauſe the greateſt difficulty in genera⯑tion is to conceive how life is firſt produced, the future expanſion and growth of the parts being only acceſſory operations; and, conſequently, that not a doubt remained of theſe animalcules being deſtined to become men, or perfect ani⯑mals, according to their ſpecies. When the im⯑probability was objected to them, that millions of animalcules, all equally capable of becoming men, ſhould be employed for this purpoſe, while only one of them was to enjoy the ſingular ad⯑vantage of being admitted into the condition of humanity; when it was demanded of them, why this uſeleſs profuſion of human germs? they replied, That it correſponded with the u⯑ſual magnificence of Nature; that, in plants and trees, millions of ſeeds were produced, while only a few of them ſucceeded; and that, therefore, we ought not to be ſurpriſed at the prodigious number of ſpermatic animals. When the ex⯑treme minuteneſs of a ſpermatic worm, com⯑pared with the body of a man, was mentioned to them as a difficulty, they anſwered, that the ſeeds of trees, of the elm, for example, were e⯑qually minute, when compared with the perfect individuals; and they added, with equal pro⯑priety, metaphyſical arguments, by which they proved, that largeneſs and minuteneſs were only relations, and that the tranſition from ſmall to great, or from great to ſmall, was performed by [136] Nature with greater facility than we could poſ⯑ſibly imagine.

Beſides, they aſked, are there not fre⯑quent examples of the transformation of in⯑ſects? Do we not daily ſee ſmall aquatic worms, by ſimply throwing off their ſkin or covering, from which they received their external figure, transformed into winged animals? May not ſpermatic animalcules, by a ſimilar transforma⯑tion, become perfect animals? Every thing, therefore, they conclude, concurs in eſtabliſhing this ſyſtem of generation, and in overturning that which is founded on the notion of eggs; and, though eggs really exiſted in viviparous animals, as well as in the oviparous, theſe eggs would only be the matter neceſſary for the growth and expanſion of the ſpermatic worm, which enters by the pedicle that attaches the egg to the ovarium, where it finds abundance of nouriſhment prepared for it. All the worms which are ſo unfortunate as to miſs this paſſage through the pedicle into the egg, periſh, and that one alone which finds the proper road, is tranſ⯑formed into a perfect animal. The difficulty of finding this paſſage is ſufficient to account for the great number and apparent profuſion of the ſpermatic animals. It is a million to one againſt any individual worm's finding this paſſage; but, to compenſate this difficulty, there are more than a million of worms. When a worm has once got poſſeſſion of an egg, no other can enter into [137] it; becauſe, ſay they, the firſt worm ſhuts up the paſſage; or rather, there is a valve at the entry to the pedicle, which plays while the egg is not perfectly full; but, when the worm has filled the egg, this valve will not open, though puſh⯑ed by a ſecond worm. Beſides, this valve is exceedingly well contrived; for, if the worm ſhould chance to deſcend through the paſſage by which it entered, the valve prevents its eſcape, and obliges it to remain till it be tranſformed. The ſpermatic worm then becomes a real foetus; and it is nouriſhed by the ſubſtance of the egg, and the membranes ſerve it for a covering; and, when the nouriſhment contained in the egg be⯑gins to fail, the foetus attaches itſelf to the in⯑ternal ſurface of the uterus, and, by this means, extracts nouriſhment from the blood of the mo⯑ther, till, by its weight, and the increaſe of its ſtrength, it at laſt breaks off all connection with the uterus, and iſſues into the world.

According to this ſyſtem, it was not the firſt woman, but the firſt man, who contained all mankind in his own body. The pre-exiſtent germs are no longer inanimate embryos locked up in eggs, and included, in infinitum, within each other. They are, on the contrary, ſmall ani⯑mals, or organized living homunculi, included in each other in endleſs ſucceſſion, and which, to render them men, or perfect animals, require nothing but expanſion, and a transformation ſi⯑milar to that of winged inſects.

[138] As phyſicians are at preſent divided between the ſyſtem of ſpermatic worms, and that of eggs, and, as every new writer upon generation has adopted either the one or the other of theſe hy⯑potheſis, it is neceſſary to examine them with care, and to ſhow not only their inſufficiency to ex⯑plain the phaenomena of generation, but that they reſt upon ſuppoſitions which are entirely deſtitute of probability.

Both ſyſtems ſuppoſe an infinite progreſſion, which, as formerly remarked, is a mere illuſion of the brain. A ſpermatic worm is more than a thouſand million of times ſmaller than a man. If, then, the body of a man be taken as an unit, the body of a ſpermatic worm will be expreſſed by the fraction 1/1000000000, i. e. by a number conſiſting of ten cyphers; and, as man is to a ſpermatic worm of the firſt generation in the ſame proportion as this worm is to a worm of the ſecond generation, the ſize of this laſt ſper⯑matic worm will be expreſſed by a number con⯑ſiſting of 19 cyphers; for the ſame reaſon, the ſize of a ſpermatic worm of the third generation muſt be expreſſed by a number conſiſting of 28 cyphers, that of the fourth generation by 37 cyphers, that of the fifth generation by 46 cy⯑phers, and that of the ſixth generation by 55 cyphers. To form an idea of the minuteneſs repreſented by this fraction, let us take the di⯑menſions of the ſphere of the univerſe from the Sun to Saturn; and, ſuppoſing the Sun to be a [139] million of times larger than the Earth, and di⯑ſtant from Saturn a thouſand ſolar diameters, we ſhall find that 45 cyphers are ſufficient to expreſs the number of cubic lines contained in this ſphere; and, if we reduce each cubic line into a thouſand million of atoms, no more than 54 cyphers will be neceſſary to expreſs their number: Of courſe, a man will be proportional⯑ly greater, when compared with a ſpermatic worm of the ſixth generation, than the ſphere of the univerſe when compared to the ſmalleſt atom that can be ſeen with the aſſiſtance of a microſcope. But, if this calculation were carried on to the 16th generation, the minuteneſs would exceed all powers of expreſſion. It is apparent, therefore, that the probability of this hypotheſis vaniſhes in proportion as the object dininiſhes. This calculation applies equally to eggs as to ſpermatic worms; and the want of probability is common to both. It will, no doubt, be ob⯑jected, that, as matter is infinitely diviſible, this gradual diminution of ſize is not impoſſible. To this I reply, that all infinities, whether in geometry or in arithmetic, are only mental ab⯑ſtractions, and have no actual exiſtence in Na⯑ture. If the infinite diviſibility of matter is to be regarded as an abſolute infinite, it is eaſy to demonſtrate, that, in this ſenſe, it has no ex⯑iſtence; for, if we once ſuppoſe the ſmalleſt poſ⯑ſible atom, by the very ſuppoſition, this atom muſt be indiviſible; becauſe, if it were diviſible, [140] it would not be the ſmalleſt poſſible atom, which is contrary to the ſuppoſition. It is, therefore, apparent, that every hypotheſis which admits an infinite progreſſion ought to be rejected not only as falſe, but as deſtitute of every veſtige of probability; and, as both the vermicular and ovular ſyſtems ſuppoſe ſuch a progreſſion, they ſhould be excluded for ever from philoſophy.

Theſe ſyſtems are liable to another objection: In the ovular ſyſtem, the firſt woman contain⯑ed both male and female eggs; the male eggs could only give origin to males; but the female eggs behoved to contain millions of generations of both males and females: Hence every wo⯑man muſt have always contained a certain num⯑ber of eggs capable of being unfolded in infini⯑tum, and another number, which could only be unfolded once, and could have no farther ope⯑ration in the ſeries of exiſtence. The ſame thing muſt take place in the vermicular ſyſtem. Hence we may conclude, that there is not the ſmalleſt degree of probability in hypotheſes of this nature.

A third difficulty ſtill remains, ariſing from the reſemblance of children ſometimes to the father, ſometimes to the mother, and ſometimes to both, and from the evident characters of ſpe⯑cific differences in mules and other monſtrous productions. If the foetus proceeds from the ſpermatic worm of the father, how comes the child to reſemble its mother? If the foetus pre⯑exiſts [141] in the egg of the mother, how ſhould the child reſemble its father? And, if the ſpermatic worm of a horſe, or the egg of a ſhe-aſs, be the origin of the foetus, how ſhould the mule par⯑take of the nature and figure both of the horſe and aſs?

Theſe general objections, though perfectly invincible, are not the only difficulties with which theſe ſyſtems are embarraſſed. May it not be demanded of thoſe who embrace the ver⯑micular ſyſtem, how theſe worms are tranſ⯑formed, and wherein conſiſts the analogy be⯑tween this transformation and that which inſects undergo? The caterpillar which is to become a butterfly, paſſes through a middle ſtate, and, after it ceaſes to be a chryſalis, is completely formed, has acquired its full growth, and is inſtantly capable of generating: But, in the pretended transformation of the ſpermatic worm of a man, there is no middle or chryſalis ſtate; and, ſup⯑poſing this to happen during the firſt days of conception, why is not the production of this chryſalis, in place of an unformed embryo, a perfect adult? Here all analogy ceaſes; and, of courſe, the notion of the transformation of the ſpermatic worm can receive no ſupport from this quarter.

Beſides, the worm which is to be transformed into a flie proceeds from an egg; this egg is produced by the copulation of the male and fe⯑male, and it includes the foetus which is to paſs [142] into a chryſalis, before it arrives at the perfect ſtate of a flie, and before it acquires the power of generating. But the ſpermatic worm has no generative faculty; neither does it proceed from an egg: And, though it ſhould be ſuppoſed that the ſemen contains eggs which produce the ſper⯑matic animals, the ſame difficulty ſtill remains; for theſe ſuppoſed eggs are not a reſult of the copulation of two ſexes, like thoſe of inſects. Con⯑ſequently, the analogy fails here likewiſe; and the transformation of inſects, in place of ſtrength⯑ening this hypotheſis, ſeems entirely to deſtroy it.

The ſeeds of vegetables are reſorted to, in or⯑der to account for the infinite number of ſper⯑matic animals: But this analogy does not ap⯑ply; for, all the ſpermatic animals, one only ex⯑cepted, muſt abſolutely periſh. The ſeeds of vegetables, however, are not ſubject to the ſame neceſſity. When they become not vegetables themſelves, they nouriſh other organized bodies, and ſerve the purpoſes of growth and of repro⯑duction to animals. But the prodigious ſuper⯑fluity of ſpermatic animals can anſwer no end whatever. I make this remark, purely becauſe I wiſh to omit nothing that has been advanced on the ſubject; for I acknowledge, that no ar⯑gument drawn from final cauſes can either eſtabliſh or deſtroy a phyſical theory.

The apparent equality in the number of ſper⯑matic animalcules in all animals, has alſo been [143] objected to by the ſupporters of this doctrine. If theſe animalcules are the immediate cauſe of generation, why is there no proportion between their numbers and thoſe of the young, which are various in men, quadrupeds, birds, fiſhes, and inſects? Beſides, there is no proportional difference in moſt ſpecies of ſpermatic animals, thoſe of a rat being nearly equal in ſize to thoſe of a man. Even when a difference in ſize takes place, it has no proportion to the bulks of the animals. The ſpermatic animals of the calmar, which is a ſmall fiſh, are a hundred thouſand times larger than thoſe of a man or of a dog. This is an additional proof that theſe worms are not the ſole and immediate cauſe of genera⯑tion.

The particular objections to the ovular ſyſtem are not leſs weighty. If the foetus exiſted in the egg before the junction of the male and female, why do we not ſee the foetus in the egg before impregnation, as clearly as after it? We formerly mentioned, that Malpighius always found the foetus in eggs which had received the impregnation of the male, and could diſcover nothing but an unformed mole or maſs in the cicatrice of unimpregnated eggs. It is, there⯑fore, evident, that the foetus is never formed till the egg has been impregnated.

Farther, we not only cannot diſcover the foe⯑tus in eggs before the intercourſe of the ſexes, but we have not been able to demonſtrate the [144] exiſtence of eggs in viviparous animals. Thoſe phyſicians who pretend that the ſpermatic worm is the foetus incloſed in a coat or covering, are at leaſt aſcertained of the exiſtence of ſpermatic worms; but thoſe who maintain that the foetus pre-exiſts in the egg, have no evidence of the exiſtence of the egg itſelf; for the probability of their non-exiſtence in viviparous animals a⯑mounts almoſt to a certainty.

Though the partizans of the ovular ſyſtem agree nto as to what ought to be regarded as the real egg in the teſticles of females, they all allow, however, that impregnation is accompliſhed in the teſticles or ovarium. But they never con⯑ſider, that, if this were the caſe, moſt foetuſes would be found in the abdomen in place of the uterus; for, as the ſuperior extremity of the Fallopian tube is unconnected with the ovarium, the pretended eggs would often fall into the ab⯑domen. Now, we know this to be at leaſt a very rare phaenomenon; and it is probable that it never happens but by means of ſome violent accident.

Theſe objections and difficulties have not e⯑ſcaped the ingenious author of Venus Phyſique. But, as his work is in the hands of the public, and as it admits not of abridgement, we ſhall refer the reader to the book itſelf; and ſhall conclude with an account of a few particular experiments, ſome of which appeared to favour and others to contradict the above ſyſtems.

[145] In the hiſtory of the Academy of ſciences, ann. 1701, ſome objections are propoſed by M. Mery againſt the egg-ſyſtem. This able anato⯑miſt maintained, with propriety, that the veſicles found in the teſticles of females are not eggs; that they adhere ſo firmly to the internal ſurface of the teſticle, as not to admit of a natural ſepa⯑ration; and that, though they could ſeparate from the ſubſtance of the teſticle, it was impoſ⯑ſible for them to get out of it, becauſe the tex⯑ture of the common membrane incloſing the whole teſticle is ſo firm and ſtrong, that it is im⯑practicable to conceive the poſſibility of its being pierced by a veſicle or round ſoft egg. And, as moſt anatomiſts and phyſicians were prepoſ⯑ſeſſed in favour of the egg-ſyſtem, and imagined that the number of cicatrices in the teſticles cor⯑reſponded with the number of foetuſes, M. Mery ſhowed ſuch a quantity of theſe cicatrices in the teſticles of a woman, as, upon the ſuppoſition of the truth of this ſyſtem, would have argued a fecundity beyond the power of credibility. Theſe difficulties ſtimulated other anatomiſts of the academy, who were partizans of the eggs, to make new reſearches. M. Duverney ex⯑amined the teſticles of cows and ſheep, and maintained, that the veſicles were eggs, becauſe ſome of them adhered leſs firmly to the teſticles than others; and that it was natural to ſuppoſe that they ſeparated altogether when they arrived at full maturity; eſpecially as by blowing into [146] the cavity of the teſticle, the air paſſed between the veſicles and the adjacent parts. M. Mery ſimply replied, that this proof was inſufficient, as theſe veſicles were never ſeen ſeparate from the teſticles. M. Duverney farther remarked the glandular bodies upon the teſticles; but he never conſidered them as parts eſſential to ge⯑neration, but as accidental excreſcences, like gall-nuts on the oak. M. Littre, whoſe preju⯑dices in favour of eggs were ſtill ſtronger, maintained, not only that the veſicles were eggs, but even aſſured us, that he diſcovered in one of them a well formed foetus, of which he could diſtinguiſh both the head and trunk; and he has even given their dimenſions. But, admitting this wonder, which never appeared to any eyes excepting his own, to be convinced of the doubt⯑fulneſs of the fact, we have only to peruſe his memoir^*. From his own deſcription it appears, that the uterus was ſchirrous, and the teſticle very much corrupted; that the veſicle or egg, which contained the pretended foetus, was much leſs than [...]mmon, &c.

Nuck furniſhes us with a celebrated experi⯑ment in favour of eggs. He opened a bitch three days after copulation; he drew out one of the horns of the uterus, and tied it in the middle ſo as to prevent the ſuperior part of the Fallopian tube from having any communication with the inferior part. After this he replaced the horn of [147] the uterus, and cloſed the wound. Twenty-four days afterwards, he again opened the wound, and found two foetuſes in the ſuperior part of the tube, that is, between the teſticles and the ligature; and there was no foetus in the un⯑der part. In the other horn of the uterus, that was not tied, he found three foetuſes regularly diſpoſed; which proves, ſays he, that the foetus proceeds not from the male-ſemen, but that it exiſts in the egg of the female. Suppoſing this experiment, which is ſingle, had been often re⯑peated with the ſame ſucceſs, the concluſion the author draws from it is not legitimate: It proves no more than that the foetus may be formed in the ſuperior part of the horn of the uterus, as well as in the inferior; and it is natural to think that, by the preſſure of the ligature, the ſeminal liquor in the inferior part was forced out, and, of courſe, fruſtrated the work of generation in that region of the uterus.

This is all the length that anatomiſts and phy⯑ſicians have proceeded in the ſubject of genera⯑tion. It only remains that I deliver the reſults of my own experiments and inquiries; and I ſhall leave the reader to judge whether my ſy⯑ſtem be not infinitely more conſonant to nature than any of thoſe of which I have given an ac⯑count.

CHAP. VI.
Experiments on Generation.

I Often reflected on the above two ſyſtems of generation, and was daily more and more convinced that my theory was infinitely more probable. At length I began to ſuſpect that theſe living organic particles, from which I thought all animals and vegetables derived their origin, might be recognized, by the aſſiſtance of good glaſſes. My firſt notion was, that the ſpermatic animalcules found in the ſeminal fluid of all males, might probably be theſe very organic particles; and I reaſoned in this manner. If all animals and vegetables contain an infinite num⯑ber of organic particles, theſe particles ſhould be moſt abundant in their ſeeds, becauſe the ſeed is an extract from all the organic parts, and eſpe⯑cially from thoſe which are moſt analogous to the individual: Perhaps the ſpermatic animals found in the ſemen of males may actually be thoſe very organic particles, or at leaſt, the firſt union or aſſemblages of them. But, if this be the caſe, then the ſemen of females ought to contain or⯑ganic living particles, or animalcules, ſimilar to [149] thoſe of the male. They ought, for the ſame reaſon, to be found in the ſeeds of plants, in the nectarium, and in the ſtamina, which are the moſt eſſential parts of vegetables, and contain the or⯑ganic particles neceſſary for their reproduction. I therefore determined to examine with the microſcope the ſeminal liquors of males and fe⯑males, and the germs of plants; and, at the ſame time, I imagined that the cavities of the glandu⯑lar bodies of the uterus might be the reſervoirs of the female ſemen. Having communicated my ideas of this ſubject to my ingenious friends Mr Needham, M. Daubenton, M. Gueneau, and M. Dalibard, they encouraged me to commence a ſet of experiments in order to throw light up⯑on this myſterious operation of nature. All of theſe gentlemen occaſionally attended and aſſiſted me; but particularly M. Daubenton, who was never abſent, and who was witneſs to every ex⯑periment I made.

I employed a double microſcope, which I had from Mr Needham, being the ſame with which he made his numerous and ingenious obſerva⯑tions ^*. This inſtrument is infinitely preferable to thoſe employed by Leeuwenhoek^†.

CHAP. VII.
Compariſon of my own Experiments with thoſe of Leeuwenhoek.

THOUGH my experiments were made with all the attention of which I was ca⯑pable, and though I often repeated them, I am ſatisfied that many things muſt have eſcaped me. I have only deſcribed what I ſaw, and what e⯑very man may ſee, at the expence of a little art and patience. To free myſelf from prejudice, I even attempted to forget what other obſervers pretended to have ſeen, endeavouring, by this means, to be certain of ſeeing nothing but what really appeared; and it was not till I had di⯑geſted my experiments, that I wiſhed to compare them with thoſe of former writers, and particu⯑larly with thoſe of Leeuwenhoek, who had oc⯑cupied himſelf more than 60 years in experi⯑ments of this kind.

Whatever authority may be due to this acute obſerver, it is certainly allowable to inſtitute a compariſon between a man's own obſervations, and thoſe of the moſt reſpectable writer on the ſame ſubject. By an examination of this kind, [194] truth may be eſtabliſhed, and errors may be de⯑tected, eſpecially when the only object of in⯑quiry is to aſcertain the genuine nature of thoſe moving bodies which appear in the ſeminal fluids of all animals.

In the month of November 1677, Leeuwen⯑hoek, who had formerly communicated many microſcopic obſervations to the Royal Society of London, concerning the juices of plants, the texture of trees, the optic nerve, rain-water, &c. writes to Lord Brouncker, preſident of the So⯑ciety, in the following terms: 'Poſtquam^* Exc. Dominus Profeſſor Cranen me viſitatione ſua ſaepius honorarat, litteris rogavis Domino Ham cognato ſuo, quaſdam obſervationum mearum videndas darem. Hic Dominus Ham me ſe⯑cundo inviſens, ſecum in laguncula vitrea ſemen viri, gonorrhoea laborantis, ſponte deſtillatum, attulit, dicens, ſe poſt pauciſſimas temporis mi⯑nutias (cum materia illa jam in tantum eſſet reſoluta ut fiſtulae vitreae immitti poſſet) ani⯑malcula viva in eo obſervaſſe, quae caudam et ultra 24 horas non viventia judicabat: Idem referebat ſe animalcula obſervaſſe mortua poſt ſumptam ab aegroto therebintinam. Materi⯑am praedicatam fiſtulae vitreae immiſſam, prae⯑ſente Domino Ham, obſervavi, quaſdamque in ea creaturas viventes, ac poſt decurſum 2 aut 3 horarum eamdem ſolus materiam obſervans, mortuas vidi.'

[195] 'Eamdem materiam (ſemen virile) non ae⯑groti alicujus, non diuturna conſervatione cor⯑ruptam, vel poſt aliquot momenta fluidiorem factam, ſed ſani viri ſtatim poſt ejectionem, ne interlabentibus quidem ſex arteriae pulſibus, ſaepiuſcule obſervavi, tantamque in ea viven⯑tium animalculorum multitudinem vidi, ut in⯑terdum plura quam 1000 in magnitudine a⯑renae ſeſe moverent; non in toto ſemine, ſed in materia fluida craſſiori adhaerente, ingen⯑tem illam animalculorum multitudinem obſer⯑vavi; in craſſiori vero ſeminis materia quaſi ſine motu jacebant, quod inde provenire mihi imaginabar, quod materia illa craſſa ex tam va⯑riis cohaereat partibus, ut animalcula in ea ſe movere nequirent; minora globulis ſanguini ruborem adferentibus haec animalcula erant, ut judicem, millena millia arenam grandiorem magnitudine non aequatura. Corpora corum rotunda, anteriora obtuſa, poſteriora ferme in aculeum deſinentia habebant; cauda tenui lon⯑gitudine corpus quinquies ſexieſve excedente, et pellucida, craſſitiem vero ad 25 partem cor⯑poris habente, praedita erant, adeo ut ea quo ad figuram cum cyclaminis minoribus, longam caudam habentibus, optime comparare queam: Motu caudae ſerpentino, aut ut anguillae in aqua natantis, progrediebantur; in materia vero aliquantulum craſſiori caudam octies decieſve quidem evibrabant antequam latitudinem capilli procedebant. Interdum imaginabar me inter⯑noſcere [196] noſcere poſſe adhuc varias in corpore horum animalculorum partes, quia vero continuo eas videre nequibam, de iis tacebo. His animal⯑culis minora adhuc animalcula, quibus non niſi globuli figuram attribuere poſſum, permiſta erant.'

'Memini me ante tres aut quatuor annos, ro⯑gatu Domini Oldenburg B. M. ſemen virile ob⯑ſervaſſe, et praedicta animalia pro globulis ha⯑buiſſe; ſed quia faſtidiebam ab ulteriori inqui⯑ſitione, et magis quidem a deſcriptione, tunc temporis eam omiſi. Jam quoad partes ipſas, ex quibus craſſam ſeminis materiam, quoad majorem ſui partem conſiſtere ſaepius cum ad⯑miratione obſervavi, ea ſunt tam varia ac multa vaſa, imo in tanta multitudine haec vaſa vidi, ut credam me in unica ſeminis gutta plura ob⯑ſervaſſe quam anatomico per integrum diem ſubjectum aliquod ſecanti occurrant. Quibus viſis, firmiter credebam nulla in corpore hu⯑mano jam formato eſſe vaſa, quae in ſemine virili bene conſtituto non reperiantur. Cum materia haec per momenta quaedam aëri fuiſſet expoſita, praedicta vaſorum multitudo in aquo⯑ſam magnis oleaginoſis globulis permiſtam ma⯑teriam mutabatur,' &c.

The ſecretary of the Royal Society replied to this letter of Leeuwenhoek, that it would be proper to make ſimilar experiments on the ſemi⯑nal fluids of other animals, not only to ſupport the original diſcovery, but to diſtinguiſh what⯑ever differences might appear in the number and [197] figure of the animalcules: And, with regard to the vaſcular texture of the thick part of the ſe⯑minal fluid, he ſuſpected that it was only a con⯑geries of filaments, without any regular organi⯑zation: 'Quae tibi videbatur vaſorum congeries, fortaſſis ſeminis ſunt quaedam filamenta, haud organice conſtructa, ſed dum permearunt vaſa generationi inſervientia in iſtiuſmodi figuram elongata. Non diſſimili modo ac ſaepius no⯑tatus ſum ſalivam craſſiorem ex glandularum faucium foraminibus editam, quaſi e convolutis fibrillis conſtantem^*.'

Leeuwenhoek replied, 18th March 1678, in the following words: 'Si quando canes coeunt marem a foemina ſtatim ſeponas, materia quae⯑dam tenuis et aquoſa (lympha ſcilicet ſperma⯑tica) a pene ſolet paulatim exſtillare; hanc ma⯑teriam numeroſiſſimis animalculis repletam ali⯑quoties vidi, eorum magnitudine quae in ſe⯑mine virili conſpiciuntur, quibus particulae globulares aliquot quinquagies majores per⯑miſcebantur.'

'Quod ad vaſorum in craſſiori ſeminis virilis portione ſpectabilium obſervationem attinet, denuo non ſemel iteratam, ſaltem mihimetipſi comprobaſſe videor; meque omnino perſuaſum habeo, cuniculi, canis, felis, arterias venaſve fuiſſe a peritiſſimo anatomico haud unquam magis perſpicue obſervatas, quam mihi vaſa in ſemini virili, ope perſpicilli, in conſpectum venere.'

[198] 'Cum mihi praedicta vaſa primum innotuere, ſtatim etiam pituitam, tum et ſalivam perſpi⯑cillo applicavi; verum hic minime exiſtentia animalia fruſtra quaeſivi.'

'A cuniculorum coitu lymphae ſpermaticae guttulam unam et alteram, e femella extillan⯑tem, examini ſubjeci, ubi animalia praedicto⯑rum ſimilia, ſed longe pauciora, comparuere. Globuli item quam plurimi, plerique magnitu⯑dine animalium, iiſdem permiſti ſunt.'

'Horum animalium aliquot etiam delinea⯑tiones tranſmiſi; figura 1. [pl. VI. fig. 1.] ex⯑primit eorum aliquot vivum (in ſemine cuni⯑culi arbitror) eaque forma qua videbatur, dum aſpicientem me verſus tendit. A B C, capitu⯑lum cum trunco indicant; C D, ejuſdem cau⯑dam, quam pariter ut ſuam anguilla inter na⯑tandum vibrat. Horum millena millia, quan⯑tum conjectare eſt, arenulae majoris molem vix ſuperant. [Pl. VI. fig. 2. 3. 4.] ſunt ejuſdem generis animalia, ſed jam emortua.'

'[Pl. VI. fig. 5.] delineatur vivum animalcu⯑lum quemadmodum in ſemine canino ſeſe ali⯑quoties mihi attentius intuenti exhibuit. E F G, caput cum trunco indigitant, G H, ejuſdem caudam. [Pl. VI. fig. 6. 7. 8.] alia ſunt in ſemine canino quae motu et vita privantur, qualium etiam vivorum numerum adeo in⯑gentem vidi, ut judicarem portionem lymphae ſpermaticae arenulae mediocri reſpondentem, eorum ut minimum decena millia continere.'

[199] In another letter to the Royal Society, dated May 31. 1678, Leeuwenhoek adds, 'Seminis canini tantillum microſcopio applicatum iterum contemplatus ſum, in eoque antea deſcripta animalia numeroſiſſime conſpexi. Aqua plu⯑vialis pari quantitate adjecta, iiſdem confeſtim mortem accercit. Ejuſdem ſeminis canini por⯑tiuncula in vitreo tubulo unciae partem duo⯑decimalem craſſo ſervata, ſex et triginta hora⯑rum ſpatio contenta animalia vita deſtituta ple⯑raque, reliqua moribunda videbantur.'

'Quo de vaſorum in ſemine genitali exiſten⯑tia magis conſtaret, delineationem aliqualem mitto, ut in figura A B C D E [pl. VI. fig. 9.] quibus literis circumſcriptum ſpatium arenu⯑lam mediocrem vix ſuperat.'

I have tranſcribed theſe paſſages from the Phi⯑loſophical Tranſactions, becauſe they firſt ap⯑peared in that work, before Leeuwenhoek had formed any theory; and, therefore, they muſt be more agreeable to truth. After the ingenious author had formed a ſyſtem of generation, his account of the ſpermatic animals varies, even in matters that are eſſential^*.

Secondly, It is worthy of remark, that Leeu⯑wenhoek had very early diſcovered the filaments which appear in the ſemen before it be liqui⯑ſied; and that, at that time, when he had not framed his hypotheſis concerning the ſpermatic animals, he conceived the filaments to be veins, nerves, and arteries. He firmly believed, that all the parts and veſſels of the human body might be clearly diſtinguiſhed in the ſeminal fluid. He even perſiſted in this opinion, not⯑withſtanding the repreſentations made to him by Mr Oldenburg, in name of the Royal Society. But, after he conceived the notion of transform⯑ing his spermatic animals into men, he never again takes any notice of theſe veſſels. Inſtead of regarding them as the nerves and blood-veſ⯑ſels [203] of the human body already formed in the ſemen, he does not even aſcribe to them their real function, which is the production of the moving bodies. He obſerves^*, 'Quid fiet de omnibus illis particulis ſeu corpuſculis praeter illa animalcula ſemini virili hominum inhae⯑rentibus! Olim et priuſquam haec ſcriberem, in ea ſententia fui praedictas ſtrias vel vaſa ex teſticulis principlum ſecum ducere,' &c. And, in another place, he ſays, that what he had for⯑merly remarked concerning veſſels in the ſemen deſerved no attention.

Thirdly, If we compare the figures 1. 2. 3. and 4. Pl. VI. and VII. which we have repreſented exactly as they appear in the Philoſophical Tranſactions, with thoſe which Leeuwenhoek cauſed to be engraved ſeveral years after, we ſhall find very great differences, eſpecially in thoſe of the dead animalcules of the rabbit, 1. 3. and 4. and in thoſe of the dog, which I have alſo delineated, in order to give a diſtinct idea of the matter. From all this, it may fairly be concluded, that Leeuwenhoek has not always ſeen the ſame phaenomena; that the moving bodies, which he regards as animals, have appeared to him under different forms; and that he has contradicted himſelf with a view to make the ſpecies of men and of animals uniform and conſiſtent. He not only varies as to the fundamental part of theſe experiments, but alſo as to the manner of ma⯑king [204] them; for he expreſsly tells us, that he al⯑ways diluted the ſemen with water, to ſeparate its parts, and to give more freedom of motion to the animalcules^*; and yet, in his firſt letter to Lord Brouncker, he ſays, that, when he mixed the ſemen of dogs, in which he before had ſeen innumerable animals, with water, they all in⯑ſtantly died. Thus Leeuwenhoek's firſt experi⯑ments were made, like mine, without any mix⯑ture; and, it appears, that he was not in uſe to mix the liquor with water till long after he be⯑gan his experiments, and till he conceived the idea of water's killing the animalcules; which, however, is not true: I imagine that the addi⯑tion of water only diſſolves the filaments too ſuddenly; for, in all my experiments, I have ſeen but very few filaments in the liquor, after its being mixed with water.

Leeuwenhoek was no ſooner perſuaded that the ſpermatic animalcules were transformed in⯑to men and other animals, than he imagined that he ſaw two diſtinct kinds in the ſemen of every animal, the one male, and the other female. Without this difference of ſex in the ſpermatic animalcules, it was difficult, he ſays, to conceive the poſſibility of producing males and females by ſimple transformation. He mentions theſe male and female animalcules in his letter publiſhed in the Philoſophical Tranſactions, No. 145. and in ſeveral other places^†. But he attempts not to [205] deſcribe the differences between male and fe⯑male animalcules, which never exiſted but in his own imagination.

The famous Boerhaave having aſked Leeu⯑wenhoek, whether he had obſerved any diffe⯑rences in the growth and ſize of ſpermatic ani⯑mals? Leeuwenhoek replied, that, in the ſemen of a rabbit which he had opened, he ſaw an in⯑finite number of animalcules: 'Incredibilem,' ſays he, 'viventium animalculorum numerum conſpexerunt, cum haec animalcula ſcypho impoſita vitreo et illic emortua, in rariores or⯑dines diſparaſſent, et per continuos aliquot dies ſaepius viſu examinaſſem, quaedam ad juſtam magnitudinem nondum excreviſſe adverti. Ad haec quaſdam obſervavi particulas perexiles et oblongas, alias aliis majores, et, quantum ocu⯑lis apparebat, cauda deſtitutas; quas quidem particulas non niſi animalcula eſſe credidi, quae ad juſtam magnitudinem non excreviſſent^*.' Here we have animalcules of different ſizes, and ſome with tails, and others that had no tails, which better correſponds with my experiments than with Leeuwenhoek's ſyſtem. We only differ in a ſingle article. He conſiders the oblong bodies without tails as young animalcules which have not yet arrived at their full growth: But I, on the contrary, have ſeen theſe pretended ani⯑mals originally ſpring from the filaments with their tails or threads, which they gradually loſt.

In another letter, written the ſame year, and addreſſed to Boerhaave^*, he relates ſome farther obſervations concerning the ſemen of the ram: He tells us, that, when the liquor was put into ſeparate glaſſes and examined, he obſerved flocks of animalcules moving all in the ſame direction, and other flocks returning the contrary way. He adds: 'Neque illud in unica epididymum parte, ſed et in aliis quas praecideram partibus, obſervavi. Ad haec, in quadam paraſtatarum reſecta portione complura vidi animalcula, quae necdum in juſtam magnitudinem adoleverant; nam et corpuſcula illis exiliora et caudae triplo breviores erant quam adultis. Ad haec, caudas non habebant deſinentes in mucronem, quales tamen adultis eſſe paſſim comperio. Practerea, in quandam paraſtatarum portionem incidi, a⯑nimalculis, quantum diſcernere potui, deſtitu⯑tam, tantum illi quaedam perexiguae inerant [208] particulae, partim longiores partim breviores; ſed altera ſui extremitate craſſiunculae; iſtas particulas in animalcula tranſituras eſſe non du⯑bitabam.' From this paſſage, it is apparent that Leeuwenhoek had ſeen in this ſeminal liquor, what I have found in the ſemen of all the ani⯑mals which I examined, moving bodies that differ⯑ed in ſize, figure, and motion; and theſe cir⯑cumſtances, it is obvious, correſpond better with the notion of organic particles in motion, than with that of real animals.

It appears, therefore, that Leeuwenhoek's ob⯑ſervations, though he draws very different con⯑cluſions from them, perfectly correſpond with mine: And, though there be ſome oppoſition in the facts, I am fully perſuaded, that, whoever ſhall take the trouble of repeating the experiments, will eaſily diſcover the ſource of theſe diffe⯑rences, and find that I have related nothing but truth. To enable the reader to decide in this matter, I ſhall add a few remarks.

We do not always ſee, in the human ſemen, the filaments I have mentioned: For this pur⯑poſe, the liquor muſt be examined the moment it is extracted from the body; and even then they do not uniformly appear. When the li⯑quor is too thick, it preſents nothing but large globules, which may be diſtinguiſhed with a common lens. When examined with the mi⯑croſcope they have the appearance of ſmall oran⯑ges; they are very opaque, and one of them oc⯑cupies [209] the whole field of the microſcope. The firſt time I obſerved theſe globules, I imagined them to be foreign bodies which had fallen into the liquor. But, after examining different drops, I found that the whole liquor was compoſed of theſe large globules. I obſerved one of the largeſt and roundeſt of them for a long time. At firſt it was perfectly opaque: A little after, I perceived on its ſurface, about half way be⯑tween the centre and circumference, a beautiful coloured luminous ring, which continued more than half an hour, then gradually approached the centre, which became clear and coloured, while the reſt of the globule remained opaque. This light which illuminated the centre, reſem⯑bled that which appears in large air-bubbles. The globule now began to grow flat, and to have a ſmall degree of tranſparency: And, after obſerving it for three hours, I could perceive no other change, no appearance of motion, either internal or external. I imagined that ſome change might happen by mixing the liquor with water. The globules were indeed changed into a tranſ⯑parent homogeneous fluid, which preſented no⯑thing worthy of remark. I left the ſemen to liquify of its own accord, and examined it at the diſtance of 6, 12, and 24 hours; but found no⯑thing like life or motion. I relate this experi⯑ment to ſhow, that the ordinary phaenomena are not always to be expected in ſeminal fluids, though they be apparently ſimilar.

[210] Sometimes all the moving bodies have tails, eſpecially in the ſemen of man, and of the dog; their motion is not then very rapid, and appears to be performed with difficulty. If the liquor be allowed to dry, the tails or threads are firſt deprived of motion; the anterior extremity con⯑tinues to vibrate for ſome time, and then all mo⯑tion ceaſes. Theſe bodies may be long preſerved in this ſtate; and, if a ſmall drop of water be then poured upon them, their figure changes; they fall down into ſeveral minute globules, which appear to have a ſmall degree of motion, ſome⯑times approaching each other, and ſometimes trembling, and turning round their centres.

The moving bodies in the human ſemen, and in thoſe of the dog and bitch, reſemble each o⯑ther ſo ſtrongly, that it is not eaſy to diſtinguiſh them, eſpecially when examined immediately af⯑ter they are taken from the body of the animal. Thoſe of the rabbit appear to be ſmaller and more active. But theſe differences and reſem⯑blances proceed more from the different ſtates of the fluids during the time of examination, than from the nature of the fluids themſelves, which ought indeed to be different in different ſpecies of animals: For example, in the human fluid, I have remarked large filaments, as repre⯑ſented in Pl. 1. fig. 3. &c. and I have ſeen the moving bodies ſeparate from theſe filaments, from which they appeared to derive their origin. But I could perceive nothing of this kind in the [211] ſemen of the dog. In place of diſtinct filaments, it is generally compoſed of a compact mucilage, in which we with difficulty perceive ſome fila⯑mentous parts; and yet this mucilage gives birth to moving bodies ſimilar to thoſe in the human ſemen.

The motion of theſe bodies continues longer in the fluid of the dog, than in that of man, which enables us more eaſily to diſtinguiſh the change of form above taken notice of. The mo⯑ment the fluid iſſues from the body of the ani⯑mal, we find moſt of the animalcules poſſeſſed of tails. In 12, 24, or 36 hours afterwards, al⯑moſt the whole tails diſappear; we then per⯑ceive only oval bodies moving about, and gene⯑rally with more rapidity than at firſt.

The moving bodies are always below the ſur⯑face of the liquor. Several large tranſparent air-bubbles commonly appear on the ſurface; but they have no motion, unleſs when the li⯑quor is agitated. Below the moving bodies we often perceive others ſtill more minute: Theſe have no tails; but moſt of them move: And, in general, I have remarked, that, of the number⯑leſs globules in all theſe liquors, the ſmalleſt are generally blacker and more obſcure than the o⯑thers; and that thoſe which are extremely mi⯑nute and tranſparent have little or no motion. They ſeem likewiſe to have more ſpecific gravi⯑ty; for they are always ſunk deepeſt in the fluid.

CHAP. VIII.
Reflections on the preceding Experiments.

FROM the foregoing experiments, it ap⯑pears, that females, as well as males, have a ſeminal fluid containing bodies in motion; that theſe moving bodies are not real animals, but only organic living particles; and that theſe particles exiſt not only in the ſeminal fluids of both ſexes, but in the fleſh of animals, and in the ſeeds of vegetables. To diſcover whether all the parts of animals and all the ſeeds of plants contained moving organic particles, I made in⯑fuſions of the fleſh of different animals, and of the ſeeds of more than twenty different ſpecies of vegetables; and, after remaining ſome days in cloſe glaſſes, I had the pleaſure of ſeeing organic moving particles in all of them. In ſome they appeared ſooner, in others later. Some preſer⯑ved their motion for months, and others ſoon loſt it. Some, at firſt, produced large moving globules, reſembling animals, which changed their figure, ſplit, and became gradually ſmaller. Others produced only ſmall globules whoſe mo⯑tions were extremely rapid; and others produ⯑ced [213] filaments, which grew longer, ſeemed to vege⯑tate, and then ſwelled, and poured out torrents of moving globules. But it is needleſs to give a detail of my experiments on the infuſions of plants, eſpecially ſince Mr Needham has publiſh⯑ed his excellent and numerous obſervations on this ſubject. To this able naturaliſt I have read over the preceding treatiſe; I have often rea⯑ſoned with him on the reſemblance between the moving bodies in infuſions of the ſeeds of vege⯑tables, and thoſe in the ſeminal fluids of male and female animals. He thought my views well founded, and of ſufficient importance to merit a farther diſcuſſion. He, therefore, began to make experiments on the different parts of vege⯑tables; and I acknowledge, that he has brought the ideas I communicated to greater perfection, than could have been done by me. Of this I could give many examples: But I ſhall confine myſelf to one, becauſe I formerly pointed out the fact in queſtion, which he deſcribes in the following manner.

To aſcertain whether the moving bodies which appear in infuſions of fleſh were real animals, or only, as I had imagined, organic moving par⯑ticles, Mr Needham thought that an examination of the jelly of roaſted meat would determine the queſtion; becauſe, if they were animals, the fire would deſtroy them, and, if not, they would ſtill be perceptible, in the ſame manner as when the fleſh was raw. Having, for this purpoſe, ta⯑ken [214] the jellies of veal and of other kinds of roaſt⯑ed meat, he put them in glaſſes filled with water, and carefully corked the bottles. After ſome days infuſion, he found in the whole of the liquors an immenſe number of moving bodies. He ſhowed me ſeveral of theſe infuſions, and, among others, that of the veal jelly, which contained moving bodies very ſimilar to thoſe of man, the dog, and the bitch, after they had loſt their tails or threads. Though they changed their forms, their motions were ſo ſimilar to thoſe of animals ſwimming, that, whoever ſaw them for the firſt time, or had been ignorant of what has been formerly remarked concerning them, would certainly have apprehended them to be real ani⯑mals. I ſhall only add, that Mr Needham has eſtabliſhed, by numberleſs experiment, the exiſt⯑ence of moving organic particles in all the parts of vegetables, which confirms what I have alledged, and extends my theory concerning the compoſition and reproduction of organized be⯑ings.

It is them apparent, that all animals, whether male or female, and every ſpecies of vegetable, are compoſed of living organic particles. Theſe organic particles abound moſt in the ſeminal fluids of animals, and in the ſeeds of vegetables. Re⯑production is effected by the union of theſe or⯑ganic particles, which are detached from all parts of the animal or vegetable body, and are always ſimilar to the particular ſpecies to which [215] they belong; for their union could not be ac⯑compliſhed but by the intervention of an inter⯑nal mould, which is the efficient cauſe of the figure of the animal or vegetable, and in which the eſſence, the unity, and the continuation of the ſpecies conſiſts, and will invariably continue till the end of time.

But, before drawing general concluſions from the ſyſtem I have eſtabliſhed, ſome objections muſt be removed, which will contribute ſtill farther to illuſtrate the ſubject.

It will be demanded of me, why I deny theſe moving bodies to be animals, after they have uniformly been recognized as ſuch by every man who has examined them? It may likewiſe be aſked, How is it poſſible to conceive the na⯑ture of living organic particles, unleſs we allow them to be real animals? And to ſuppoſe an a⯑nimal to be compoſed of leſſer animals, is near⯑ly the ſame idea, as when we ſay, that an orga⯑nized body is compoſed of organic living par⯑ticles. To theſe queſtions, I ſhall endeavour to give ſatisfactory anſwers.

It is true, that almoſt all obſervers agree in regarding the moving bodies in the ſeminal fluid as real animals. But it is equally certain, both from my experiments, and thoſe of Mr Need⯑ham upon the ſemen of the calmar, that theſe moving bodies are beings more ſimple and leſs organized than animals.

[216] The word animal, in its common acceptation, repreſents a general idea, compoſed of particular ideas which we derive from particular animals. All general ideas include many different ideas, which more or leſs approach or recede from one another; and, of courſe, no general idea can be preciſe or exact. The general idea we have formed of an animal, may be derived from the particular idea of a dog, of a horſe, and of other animals, from the power of volition, which en⯑ables them to act according to their inclination, and from the circumſtances of their being com⯑poſed of fleſh and blood, from their faculty of chuſing and of taking nouriſhment, from their ſenſes, from the diſtinction of ſexes, and from their power of reproducing. The general idea, therefore, expreſſed by the word animal, includes a number of particular ideas, not one of which conſtitutes the eſſence of the general idea: For there are animals which have no intelligence, no will, no progreſſive motion, no fleſh or blood, and appear to be only a maſs of congealed mu⯑cilage: There are others, which cannot ſeek for their food, and only receive it from the element in which they exiſt; others have no ſenſes, not even that of feeling, at leaſt in a perceptible de⯑gree. Some have no ſexes, or have both in one individual. There remains nothing, there⯑fore, in the properties of an animal, but the power of reproduction, which is common to both the vegetable and animal. It is from the [217] whole taken together that a general idea is formed; and, as this whole is compoſed of dif⯑ferent parts, there muſt of neceſſity be degrees or intervals between theſe parts. An inſect, in this ſenſe, is leſs an animal than a dog, an oyſter than an inſect, and a ſea-nettle, or a freſh water polype, than an oyſter: And, as Nature proceeds by inſenſible degrees, we ſhould find beings par⯑taking of ſtill leſs animation than a ſea-nettle or a polype. Our general ideas are only artificial methods of collecting a number of objects un⯑der one point of view; and they have, like o⯑ther artificial methods, the defect of not being able to comprehend the whole. They are in direct oppoſition to the procedure of Nature, which is uniform, inſenſible, and always parti⯑cular. It is to graſp a number of particular ideas under one word, of which we have no clearer notion than that word conveys; becauſe, when the word is once received, we imagine it to be a line drawn between the different pro⯑ductions of nature; that every thing above this line is an animal, and every thing below it a ve⯑getable, which is another word equally general, and employed as a line of ſeparation between organized bodies and brute matter. But, as has already been remarked, theſe lines of ſeparation have no exiſtence in nature. There are bodies which are neither animals, vegetables, nor mi⯑nerals, and every attempt to arrange them un⯑der either of theſe claſſes muſt be ineffectual. [218] For example, Mr Trembly, when he firſt exa⯑mined the freſh water polypus, ſpent much time before he could determine whether it was an animal or a vegetable. The reaſon is plain; this polypus is perhaps neither the one nor the other; and all that can be ſaid is, that it has moſt reſemblance to an animal: And, as we are inclined to think, that every living being is ei⯑ther an animal or a plant, we believe not the ex⯑iſtence of any organized body, unleſs it falls un⯑der ſome of theſe general denominations, al⯑though there muſt be, and in fact there are, many beings which belong neither to the one nor the other. The moving bodies found in the ſeminal fluids, and in infuſions of the fleſh of animals, as well as in thoſe of all parts of ve⯑getables, are of this ſpecies: We can neither rank them under animals nor vegetables; and no man in his ſenſes will ever maintain them to be minerals.

We may, therefore, pronounce, without heſi⯑tation, that the great diviſion of natural pro⯑ductions into animals, vegetables, and minerals, comprehends not all material beings; ſince be⯑ings exiſt which can be included in neither of theſe claſſes. Nature paſſes, by imperceptible ſteps, from the animal to the vegetable; but, from the vegetable to the mineral, the paſſage is ſudden, and the interval great. Here the law of imperceptible degrees ſuffers a violation. This circumſtance made me ſuſpect, that, by exami⯑ning [219] Nature more cloſely, we ſhould find inter⯑mediate organized beings, which, without ha⯑ving the faculty of reproduction, like animals and vegetables, would ſtill enjoy a ſpecies of life and motion; beings which, without being either animals or vegetables, might enter into the con⯑ſtitution of both; and, laſtly, beings which would conſiſt of the firſt aſſemblages of the or⯑ganic particles mentioned in the preceding chapters.

Eggs conſtitute the firſt claſs of this ſpecies of beings. Thoſe of hens and other female birds are attached to a common pedicle, and derive their nouriſhment and growth from the body of the animal. But, when attached to the ova⯑rium, they are not properly eggs; they are only yellow globes, which ſeparate from the ova⯑rium as ſoon as they acquire a certain magnitude: Such is their internal organization, however, that they abſorb nouriſhment from the lymph contained in the uterus, and convert it into the white, membranes, and ſhell. Thus the egg poſ⯑ſeſſes a ſpecies of life and organization. It grows and aſſumes a form by its own peculiar powers: It neither lives like an animal, nor vegetates like a plant, nor enjoys the faculty of reproduction. The egg, therefore, is a diſtinct being, which can neither be ranked with the animal nor mi⯑neral kingdoms. If it be alledged, that the egg is only an animal production deſtined for the nouriſhment of the chick, and ought to be re⯑garded [220] as a part of the hen; I reply, that eggs, whether impregnated or not, are always organi⯑zed in the ſame manner; that impregnation changes only a part which is almoſt inviſible; that it grows, and acquires a uniform figure and ſtructure, both externally and internally, inde⯑pendent of impregnation; and, conſequently, it ought to be conſidered as a ſeparate and di⯑ſtinct being.

This will be ſtill more apparent, if we attend to the growth and formation of the eggs of fiſhes. When the female depoſits them in the water, they are properly but the rudiments of eggs, which, being lately ſeparated from the body of the animal, attract and aſſimilate thoſe particles that are fitted for their nouriſhment; and thus increaſe in ſize by mere abſorption. In the ſame manner as the egg of the hen ac⯑quires its white and membranes while floating in the uterus, the eggs of fiſhes acquire their white and membranes in the water; and, whether they are fecundated by the male's ſhedding his milt upon them, or they remain un⯑impregnated, they ſtill arrive at full perfection. It is plain, therefore, that eggs in general ought to be regarded as organized bodies, and forming a diſtinct genus from animals and vegetables.

The organized bodies found in the ſemen of all animals, and which, like thoſe in the milt of the calmar, are natural machines, and not ani⯑mals, form a ſecond ſpecies of the ſame genus. [221] They are properly the firſt aſſemblages of thoſe organic particles ſo often mentioned; and, per⯑haps, they are the conſtituent particles of all a⯑nimated bodies. They appear in the ſemen of every animals, becauſe the ſemen is only the re⯑ſidue of the organic particles which the ani⯑mal takes in with its food. The particles, as formerly remarked, aſſimilated from the food, are thoſe which are moſt organized, and the moſt analogous to the animal itſelf: It is of theſe particles that the ſemen conſiſts; and, of courſe, we ought not to be ſurpriſed to find or⯑ganized bodies in that fluid.

To be ſatisfied that theſe organized bodies are not real animals, we have only to reflect upon the preceding experiments. The moving bo⯑dies in the ſemen have been conſidered as real animals, becauſe they have a progreſſive motion, and ſomething ſimilar to tails. But, after attend⯑ing, on the one hand, to the nature of this mo⯑tion, which is ſuddenly finiſhed, and never again commences, and, on the other, to the nature of the tails, which are only threads adhering to the moving body, we will begin to heſitate; for an animal goes ſometimes ſlow, and ſometimes faſt; and it ſometimes ſtops, and repoſes, without moving at all. Theſe moving bodies, on the contrary, go always in the ſame direction at the ſame time; I never ſaw them ſtop and again begin to move; and, if they once ſtop, it is for ever. I demand, if this continued motion, with⯑out [222] any repoſe, is common to animals; and if, from this circumſtance, we ought not to doubt concerning the real animation of theſe moving bodies? An animal ſhould always have a uni⯑form figure, as well as diſtinct members: But theſe moving bodies change their figure every moment; they have no diſtinct members; and their tails are only adventitious matter, and no part of the individual. How, then, can they be eſteemed real animals? In ſeminal liquors, we ſee ſilaments which ſtretch out, and ſeem to ve⯑getate; then they ſwell and produce moving bo⯑dies. Theſe filaments are, perhaps, of a vege⯑table nature; but the moving bodies which proceed from them cannot be animals; for we have no example of vegetables giving birth to animals. Moving bodies are found in all animal and ve⯑getable ſubſtances promiſcuouſly. They are not the produce of generation. They have no uni⯑formity of ſpecies. They cannot, therefore, be either animals or vegetables. As they are to be met with in every part of animals and of vege⯑tables, but are moſt abundant in their ſeeds, is it not natural to regard them as the organic living particles of which animals and vegetables are compoſed, as particles which, being endowed with motion, and a ſpecies of life, ought to produce, by their union, moving and living beings, and, in this manner, form animals and vegetables?

[223] But, to remove every doubt upon this ſubject, let us attend to the obſervations of others. Can the active machines diſcovered by Mr Needham in the milt of the calmar be regarded as animals? Can we believe that eggs, which are active ma⯑chines of another ſpecies, are alſo animals? If we examine Leeuwenhoek's repreſentations of the moving bodies found in many different ſub⯑ſtances, will we not be ſatisfied, at the firſt in⯑ſpection, that they are not animals, ſince none of them have any members, but are uniformly either round or oval? If we attend to what this famous obſerver has remarked concerning the motion of theſe pretended animals, we muſt be convinced that he was wrong in regarding them as real animals, and we will be more and more confirmed in the opinion, that they are only organic moving particles. We ſhall give ſome examples. Leeuwenhoek^*, gives the figure of the moving bodies in the ſeminal fluid of a male frog. This figure repreſents nothing but a thin, long body, pointed at one of the extremities. Let us attend to what he ſays concerning it: 'Uno tempore caput, (that is, the largeſt ex⯑tremity of the moving body), craſſius mihi apparebat alio; plerumque agnoſcebam ani⯑malculum haud ulterius quam a capite ad medium corpus, ob caudae tenuitatem, et cum idem animalculum paulo vehementius move⯑retur (quod tamen tarde fiebat) quaſi volumine [224] quodam circa caput ferebatur. Corpus fere carebat motu, cauda tamen in tres quatuorve flexus volvebatur.' Here we have the change of figure that I had obſerved, the mucilage from which the moving bodies with difficulty diſengage themſelves, the ſlowneſs of their mo⯑tion before they eſcape from the mucilage, and, laſtly, a part of the pretended animal in mo⯑tion while the other is dead; for, a little after⯑wards, he obſerves, 'movebant poſteriorem ſo⯑lum partem, quae ultima, morti vicinia eſſe judicabam.' All this is repugnant to the na⯑ture of an animal, but exactly correſponds with my experiments, excepting that I never ſaw the tail move but in conſequence of an agitation of the body. Speaking of the ſeminal fluid of the cod, he ſays^*, 'Non eſt putandum omnia ani⯑malcula in ſemine aſelli contenta uno eodem⯑que tempore vivere, ſed illa potius tantum vi⯑vere quae exitui ſeu partui viciniora ſunt, quae et copioſiori humido innatant prae reliquis vita carentibus, adhuc in craſſa materia, quam hu⯑mor eorum efficit, jacentibus.' If theſe are animals, why were they not all alive? Why did thoſe only live which were in the moſt fluid part of the liquor? Leeuwenhoek did not obſerve, that the thick part, inſtead of being a humor produced by the animalcules, is a mucilage which gives birth to them. If he had diluted the mucilage with water, he would at once have [225] given life and motion to the whole. The mu⯑cilage itſelf is often nothing elſe than a maſs of thoſe bodies, which begin to move as ſoon as they can diſengage themſelves; and, of courſe, this thick matter or mucilage, inſtead of being a humour produced by the animalcules, is only a congeries of the animals themſelves, or rather, as formerly remarked, the matter of which they are formed. Speaking of the ſemen of the cock, Leeuwenhoek, in his letter to Grew, ſays^*, 'Contemplando materiam (ſeminalem) animad⯑verti ibidem tantam abundantiam viventium animalium, ut ea ſtuperem; forma ſeu externa figura ſua noſtrates anguillas fluviatiles refe⯑rebant, vehementiſſima agitatione movebantur; quibus tamen ſubſtrati videbantur multi et ad⯑modum exiles globuli, item multae plan-ova⯑les figurae, quibus etiam vita poſſet attribui, et quidem propter carumdem commotiones; ſed exiſtimabam omnes haſce commotiones et agitationes provenire ab animalculis, ſicque etiam res ſe habebat; attamen ego non opi⯑nione ſolum, ſed etiam ad veritatem mihi per⯑ſuadeo has particulas, planam et ovalem ſigu⯑ram habentes, eſſe quaedam animalcula inter ſe ordine ſuo diſpoſita et mixta, vitaque adhuc carentia.' Here we have animalcules, in the ſame ſeminal fluid, of different forms; and I am convinced, from my own experiments, that, if Leeuwenhoek had obſerved thoſe oval bodies [226] with attention, he would have perceived that they moved with their own proper force, and, conſequently, that they were as much alive as the others. This change of figure, it is true, exactly correſponds with what I had obſerved: But it does not indicate a uniform ſpecies of a⯑nimals; for, in the preſent example, if the bo⯑dies having the figure of a ſerpent were genuine ſpermatic animalcules, each of which was deſtined to become a cock, and therefore implies a uni⯑form and invariable organization, what was the end and deſtination of thoſe of an oval figure? He, indeed, afterwards remarks, that theſe oval bodies might be the ſame with the ſerpentine, if we ſuppoſe them rolled up in a ſpiral manner. But ſtill, how is it poſſible to conceive that an animal, with its body in this reſtrained poſture, ſhould be able to move without extending itſelf? I, therefore, maintain, that theſe oval bodies were only the organic particles ſeparated from their threads or tails, and that the ſerpentine bodies were the ſame particles which had not yet been deprived of theſe appendages, as I have often remarked in other ſeminal fluids.

Beſides, Leeuwenhoek, who believed all theſe moving bodies to be real animals, who eſtabliſh⯑ed a ſyſtem upon that foundation, and who af⯑firmed, that ſpermatic animalcules were tranſ⯑formed into men and other animals, now ſuſpec⯑ted them to be only natural machines, or moving organic particles. He never entertained a doubt, [227] but that theſe animalcules contained the large animal in miniature. He remarks^*, 'Progene⯑ratio animalis ex animalculo in ſeminibus maſ⯑culinis omni exceptione major eſt; nam, etiamſi in animalculo ex ſemine maſculo, unde ortum eſt, figuram animalis conſpicere nequeamus, attamen ſatis ſuperque certi eſſe poſſumus figuram animalis ex qua animal ortum eſt, in animalculo quod in ſemine maſculo re⯑peritur, concluſam jacere ſive eſſe: Et quan⯑quam mihi ſaepius, conſpectis animalculis in ſemine maſculo animalis, imaginatus fue⯑rim me poſſe dicere, en ibi caput, en ibi humeros, en ibi femora; attamen cum ne mi⯑nima quidem certitudine de iis judicium ferre potuerim, hucuſque certi quid ſtatuere ſuper⯑ſedeo, donec tale animal, cujus ſemina maſcula tam magna erunt, ut in iis figuram creaturae ex qua provenit agnoſcere queam, invenire ſecunda nobis concedat fortuna.' This op⯑portunity, ſo much deſired by Leeuwenhoek, happily occurred to Mr Needham. The ſper⯑matic animals of the calmar are three or four lines in length, and are viſible without the aſſiſt⯑ance of the microſcope. Their whole parts and organization are eaſily perceived. But they are by no means ſmall calmars, as Leeuwen⯑hoek imagined. They are not even animated, though they have motion, but are only machines, [228] which ought to be regarded as the firſt union of the organic particles.

Though Leeuwenhoek had not this opportu⯑nity of undeceiving himſelf, he had, however, obſerved other appearances which ought to have had this effect. He had remarked, for example, that the ſpermatic animals of the dog^*, often changed their figure, eſpecially when the fluid was nearly evaporated; that, when dead, they had an opening in the head, which did not ap⯑pear when they were alive; and that the head was full and round, during the life of the pre⯑tended animal, and flat and ſunk after its death; Theſe circumſtances ſhould have led him to he⯑ſitate concerning the real animation of theſe bodies, and to think that the phaenomena cor⯑reſponded more with a machine which emptied itſelf, like that of the calmar, than with the pro⯑perties of an animal.

I have ſaid that the motion of theſe moving bodies, theſe organic particles, is not ſimilar to the motion of animals, and that there is no in⯑tervals in their movements. Leeuwenhoek, in tom. 1. p. 168. makes preciſely the ſame remark: 'Quotieſcunque,' ſays he, 'animalcula in ſe⯑mine maſculo animalium fuerim contemplatus, attamen illa ſe unquam ad quietem contuliſſe, me nunquam vidiſſe, mihi dicendum eſt, ſi mo⯑do ſat fiuidae ſupereſſet materiae in qua ſeſe commode movere poterant; at eadem in con⯑tinuo [229] manent motu, et tempore quo ipſis mori⯑endum appropinquante, motus magis magiſque deficit, uſquedum nullus prorſus motus in illis agnoſcendus ſit.' It is difficult to conceive, that animals ſhould exiſt, which, from the mo⯑ment of their birth to their diſſolution, ſhould continue to move rapidly, without the ſmalleſt interval of repoſe; or to imagine that the ſper⯑matic animals of the dog, which Leeuwenhoek perceived to be as active on the ſeventh day as the moment they proceeded from the body of the dog, ſhould be able, during all this time, to move with a celerity which no animal on earth could perſiſt in for a ſingle hour, eſpecially when the reſiſtence ariſing from the denſity and tena⯑city of the fluid is taken into conſideration. This ſpecies of continued motion, on the contrary, has an exact correſondence to the nature of the organic particles, which, like artificial machines, produce their effects by a continued operation, and ſtop immediately after the end is accom⯑pliſhed.

In the numerous experiments made by Leeu⯑wenhoek, he doubtleſs obſerved ſpermatic animals without tails. He even mentions them in ſome places, and endeavours to explain the phaeno⯑menon. For example, ſpeaking of the ſemen of the cod, he ſays^*, 'Ubi vero ad lactium ac⯑cederem obſervationem, in iis partibus quas a⯑nimalcula eſſe cenſebam, neque vitam neque [230] caudam dignoſcere potui; cujus rei rationem eſſe exiſtimabam, quod quamdiu animalcula natando loca ſua perfecte mutare non poſſunt, tam diu etiam cauda concinne circa corpus ma⯑neat ordinata, quodque ideo ſingula animalcula rotundum repraeſentent corpuſculum.' It would have been more ſimple, and more agreeable to truth, to have ſaid, that the ſpermatic animals of this fiſh ſometimes have tails, and ſometimes have none, than to ſuppoſe that the tails were ſo exactly would round their bodies as to give them a ſpherical figure. One would be apt to think, that Leeuwenhoek had never fixed his eyes upon, or deſcribed any moving but thoſe which had tails; he has given figures of none that wanted tails, becauſe, though they moved, he did not regard them as animals. This is the reaſon why all Leeuwenhoek's figures of ſper⯑matic animals are very ſimilar, and all drawn with tails. When they appeared in any other form, he thought they were imperfect, or rather that they were dead. Beſides, it is apparent from my experiments, that, inſtead of unfolding their tails, wherever they are placed in circumſtances proper for ſwimming, as Leeuwenhoek inſiſts, theſe pretended animals gradually loſe their tails, in proportion to the rapidity of their motions, till, at laſt, theſe tails, which are bodies foreign to the animalcules, or threads which they drag after them, totally diſappear.

[231] Leeuwenhoek, ſpeaking of the ſpermatic ani⯑mals of man^*, ſays: 'Aliquando etiam ani⯑madverti inter animalcula particulas quaſdam minores et ſubrotundas; cum vero ſe ea aliquo⯑ties eo modo oculis meis exhibuerint, ut mihi imaginarer eas exiguis inſtructas eſſe caudis, cogitare coepi annon hae forte particulae fo⯑rent animalcula recens nata; certum enim mihi eſt ea etiam animalcula per generationem pro⯑venire, vel ex mole minuſcula ad adultam pro⯑cedere quantitatem: Et quis ſeit annon ea animalcula, ubi moriuntur, aliorum animalcu⯑lorum nutritioni atque augmini inſerviant!' It appears from this paſſage, that Leeuwenhoek had ſeen, in the human ſemen, animalcules without tails; and that he is obliged to ſuppoſe them to be recently born, which is directly the reverſe of what I have obſerved; for the moving bodies are never larger than when they ſeparate from the filaments, which is the period that their mo⯑tion begins: But, as ſoon as they are fully diſen⯑gaged from the mucilage, they become ſmaller, and continue to diminiſh till their motion entire⯑ly ceaſes. With regard to the generation of theſe animals, which Leeuwenhoek imagines to be cer⯑tain, no veſtige of copulation has been diſcerned by the moſt acute obſervers. It is purely a ran⯑dom aſſertion, as may be eaſily proved from his own experiments. He remarks, for example, [232] with great propriety, that the milt of the cod^* is gradually filled with ſeminal liquor; and that, after the fiſh has ſpent this liquor, the milt dries, and leaves only a flaccid membrane, entirely deſtitute of every kind of fluid. 'Eo tempore,' ſays he, 'quo aſellus major lactes ſuos emiſit, rugae illae, ſeu tortiles lactium partes, uſque adeo con⯑trahuntur, ut nihil praeter pelliculas ſeu membra⯑nas eſſe videantur.' How ſhould this dry mem⯑brane, which contains neither ſeminal liquor nor animalcules, produce animalcules of the ſame ſpe⯑cies next ſeaſon? If they were produced by a re⯑gular generation, ſuch a long interruption could not take place, which, in moſt fiſhes, continues a whole year. To remove this difficulty, he af⯑terwards remarks: 'Neceſſario ſtatuendum erit, ut aſellus major ſemen ſuum emiſerit, in lacti⯑bus etiamnum multum materiae ſeminalis gig⯑nendis animalculis aptae remanſiſſe, ex qua ma⯑teria plura oportet provenire animalcula ſemi⯑nalia quam anno proxime elapſo emiſſa fuerant.' This ſuppoſition, that part of the ſeminal liquor remains in the milt for the production of ſper⯑matic animals the following year, is perfectly gratuitous, and contrary to obſervation; for the milt, during this interval, is nothing but a thin dry membrane. But how will he explain a phaenomenon that takes place in ſome fiſhes, and particularly in the calmar, whoſe ſeminal liquors are not only renewed every year, but even the [233] membranes which contain them. Here neither the milt nor the ſeminal liquor are preſerved till the ſucceeding year; and, of courſe, their regular reproduction cannot be aſcribed to generation. It is, therefore, apparent, that theſe pretended ſpermatic animals are not multiplied, like other animals, by generation; and this circumſtance alone would entitle us to conclude, that the mo⯑ving particles in the ſeminal fluid are not real animals. Leeuwenhoek, though he tells us, in the paſſage above quoted, that the ſpermatic a⯑nimals are certainly propagated by generation, acknowledges, however, in another place^*, that the manner in which theſe ſpermatic animals are produced, is exceedingly obſcure, and that he leaves to others the farther elucidation of this ſubject. 'Perſuadebam mihi,' ſays he, ſpeaking of the ſpermatic animals of the dormouſe, 'haecce animalcula ovibus prognaſci, quia diverſa in orbem jacentia et in ſemet convoluta videbam; ſed unde, quaeſo, primam illorum originem de⯑rivabimus! an animo noſtro concipiemus ho⯑rum animalculorum ſemen jam procreatum eſſe in ipſa generatione, hocque ſemen tam diu in teſticulis hominum haerere, uſquedum ad annum aetatis decimum-quartum vel decimum-quintum aut ſextum pervenerint, eademque animalcula tum demum vita donari, vel in juſtam ſtaturam excreviſſe, illoque temporis articulo generandi maturitatem adeſſe! ſed haec [234] lampada aliis trado.' It is, perhaps, unneceſ⯑ſary to make many remarks on what Leeuwen⯑hoek has here advanced. He ſaw, in the ſemen of the dormouſe, ſpermatic animals which were round and without tails; in ſemet convoluta, ſays he, becauſe he always ſuppoſes that they ought to have tails. He was formerly certain that theſe animals were propagated by genera⯑tion: Here he ſeems to be convinced of the re⯑verſe. But, when he learned, that the vine⯑fretters (pucerons) were propagated without co⯑pulation ^*, he laid hold of this idea, in order to explain the generation of ſpermatic animals. 'Quemadmodum,' ſays he, 'animalcula haec quae pediculorum antea nomine deſignavimus (the pucerons) dum adhuc in utero materno latent, jam praedita ſunt materia ſeminali ex qua ejuſdem generis proditura ſunt animalcula, pari ratione cogitare licet animalcula in ſemi⯑nibus maſculinis ex animalium teſticulis non migrare, ſeu ejici, quin poſt ſe relinquant mi⯑nuta animalcula, aut ſaltem materiam ſemina⯑lem ex qua iterum alia ejuſdem generis ani⯑malcula proventura ſunt, idque abſque coitu, eadem ratione qua ſupradicta animalcula ge⯑nerari obſervavimus.' This ſuppoſition is not more ſatisfactory than the preceding; for, by thus comparing the generation of ſpermatic a⯑nimalcule with that of the vine-fretter, we diſ⯑cover not the reaſon why they are never ſeen in [235] the human ſemen till the age of 14 or 15; nor do we learn whence they proceed, or how they are yearly renewed in fiſhes, &c. Notwith⯑ſtanding all the efforts of Leeuwenhoek to eſta⯑bliſh the generation of ſpermatic animals, he leaves the ſubject in the greateſt obſcurity, where it probably would have for ever remained, if we had not diſcovered, by the preceding expe⯑riments, that they are not animals, but organic moving particles, contained originally in the food, and found in vaſt numbers in the ſeminal liquors of animals, which are the moſt pure and moſt organic extracts derived from the food.

Leeuwenhoek acknowledges, that he did not always find animalcules in the male ſemen; for example, in that of the cock, which he often examined, he never but once ſaw the eel-like animalcules: And, ſome years after, he could not diſcover theſe eels^*, but found animalcules with a large head and a tail, which his drawer could not perceive. He likewiſe remarks, that, during one ſeaſon, he could not diſcover living animals in the ſeminal fluid of the cod^†. All theſe diſappointments proceeded from this cir⯑cumſtance, that, though he ſaw moving glo⯑bules, he was unwilling to acknowledge them to be animals, unleſs they had tails, though it is in the form of globules that they moſt generally appear, either in ſeminal fluids, or in infuſions [236] of animal and vegetables ſubſtances. In the ſame place, he remarks, that, though he had of⯑ten diſtinctly ſeen the ſpermatic animals of the cod, he was never able to make his drawer per⯑ceive them: 'Non ſolum,' ſays he, 'ob eximi⯑am eorum exilitatem, ſed etiam quod eorum corpora adeo eſſent fragilia, ut corpuſcula paſ⯑ſim dirumperentur; unde factum fuit ut non⯑niſi raro, nec ſine attentiſſima obſervatione, ani⯑madverterem particulas planas atque ovorum in morem longas, in quibus ex parte caudas dignoſcere licebat; particulas has oviformes ex⯑iſtimavi animalcula eſſe dirupta, quod particu⯑lae hae diruptae quadruplo fere viderentur majores corporibus animalculorum vivorum.' When an animal, whatever be its ſpecies, dies, it does not ſuddenly change its form; from being long like a thread, it does not become round like a bullet; neither does it become four times as large after death as before it. Not a ſingle ar⯑ticle of what is advanced by Leeuwenhoek, in this paſſage, has the ſmalleſt correſpondence to the nature of an animal; but, on the contrary, the whole agrees with a ſpecies of machines which, like thoſe of the calmar, burſt and empty themſelves, after having performed their func⯑tions. To purſue this obſervation a little fur⯑ther: He tells us, that he has ſeen the ſpermatic animals of the cod under different forms, 'Mul⯑ta apparebant animalcula ſphaeram pellucidam repraeſentantia, and of different ſizes,' 'Haec [237] animalcula minori videbantur mole, quam ubi eadem antehac in tubo vitreo rotundo exami⯑naveram.' This is an evident proof, that there is nothing like a uniform and invariable ſpecies in theſe animalcules, and, conſequently, that they are not animals, but only organic moving par⯑ticles, which, by their different combinations, aſſume various figures and ſizes. Of theſe or⯑ganic particles, vaſt numbers appear in the ex⯑tract and in the reſidue of our food. The mat⯑ter which adheres to the teeth, and which, in healthy perſons, has the ſame ſmell with the ſe⯑minal fluid, is only a reſidue of our food. In it we accordingly find a great quantity of theſe pretended animals, ſome of which have tails, and reſemble thoſe of the ſeminal fluid. Mr Baker has given figures of four ſpecies of them, which are all a kind of cylinders, ovals, or glo⯑bules, ſome of them having tails, and others not. But, after the ſtricteſt examination, I am per⯑ſuaded, that none of them are real animals, and that they are only, like thoſe in the ſemen, the organic living particles of the food appearing under different forms. Leeuwenhoek, who knew not how to account for theſe pretended animals in the matter adhering to the teeth, ſup⯑poſes them to proceed from certain ſpecies of food, as cheeſe, in which they previouſly exiſt⯑ed; but they are found among the teeth of e⯑very perſon, whatever kind of food be eaten; and, beſides, they have no reſemblance to mites, [238] or other animalcules which appear in corrupted cheeſe. In another place, he tells us that theſe teeth-animals proceed from the ciſtern-water which we drink, becauſe he obſerved ſimilar ani⯑mals in rain-water, eſpecially when it had ſtag⯑nated upon leaden roofs. But, when we give the hiſtory of microſcopic animals, we ſhall de⯑monſtrate, that moſt of thoſe found in rain-water are only organic moving particles, which divide, unite, change their ſize and figure, and, in a word, which can be made to move or to reſt, to live or die, as often as we pleaſe.

Moſt ſeminal fluids ſpontaneouſly dilute, or be⯑come more liquid, when expoſed to the air, or to a certain degree of cold, than when they iſſue from the body. But they thicken upon the ap⯑plication of a moderate degree of heat. I expo⯑ſed ſome of theſe fluids to a degree of cold equal to that of water juſt beginning to freeze; but the pretended animalcules ſuffered not the leaſt in⯑jury from it. They moved with equal activity, and during the ſame length of time, as thoſe to which no cold had been applied. But thoſe which were expoſed to a ſmall degree of heat, ſoon ceaſed to move, becauſe the liquor thickened. If thoſe moving bodies were animals, they dif⯑fered in their nature and conſtitution from all o⯑thers, to whom a moderate degree of heat com⯑municates force and motion, and upon whoſe bodies cold has the very oppoſite effects.

[239] Before leaving this ſubject, upon which I have, perhaps, dwelt too long, I muſt ſtill add another remark, which may lead to ſome uſe⯑ful concluſions. Theſe pretended ſpermatic animals, which are nothing but the organic living particles of food, exiſt, not only in the ſeminal fluids of both ſexes, and in the remnants of food that adhere to the teeth, but likewiſe in the chyle and in the excrements. Leeuwenhoek, having met with them in the ex⯑crements of frogs, and of other animals which he diſſected, was at firſt greatly ſurpriſed; and, not being able to conjecture from whence ani⯑mals could proceed ſo ſimilar to thoſe in the ſe⯑minal liquor he had juſt been examining, he accuſes his own want of dexterity, and ſuppoſes, that, in diffecting the animal, he had inadver⯑tantly opened the ſeminal veſſels, and that the ſemen had in this manner been mixed with the faeces. But, having afterwards obſerved the ſame phaenomenon in the faeces of other ani⯑mals, and even in his own, he was then totally non-pluſſed. It is worthy of remark, that Leeuwenhoek never found animalcules in his own faeces, but when they were liquid. When⯑ever his ſtomach was out of order, and his belly was looſe, the animalcules appeared; but, when his food was properly concocted, and his faeces were hard, not a ſingle animalcule was to be found, although he diluted the faeces with wa⯑ter. Theſe facts ſeem perfectly to coincide [240] with what we formerly advanced; for, when the ſtomach and inteſtines properly perform their functions, the faeces are only the groſs dregs of the aliment, and all the nouriſhing and organic particles are abſorbed by the lacteal veſ⯑ſels: In this caſe, we cannot expect to find or⯑ganic particles in the faeces, which are ſolely compoſed of the uſeleſs and inert part of our food. But, when the ſtomach and inteſtines, from any indiſpoſition, allow the food to paſs without being properly digeſted, then the orga⯑nic particles mix with the faeces; and, when examined with the microſcope, we diſcover them in the form of living organic bodies. Hence we may conclude, that people who are troubled with looſeneſs ſhould have leſs ſeminal liquor, and be leſs fitted for the purpoſes of generation, than thoſe of a contrary habit of body.

I have all along ſuppoſed that the female fur⯑niſhes a fluid equally neceſſary to generation as that of the male. In the firſt chapter, I endea⯑voured to prove, that every organized body con⯑tains living organic particles; and, in Chap. II. and III. that nutrition and reproduction are ef⯑fects of the ſame cauſe; that nutrition is per⯑formed by abſorption, or an intimate penetration of organic particles through all parts of the body; and that reproduction is effected by the ſuperplus of theſe ſame organic particles, col⯑lected from every part of the body, and depoſit⯑ed in reſervoirs deſtined for that purpoſe. In [241] Chap. IV. I have ſhown how this theory applies to the generation of man, and other animals which have different ſexes. Females being orga⯑nized bodies, as well as males, they muſt alſo have ſome reſervoirs for the reception of the ſurplus of organic particles returned from all parts of their bodies. This ſurplus, as it is ex⯑tracted from every part of the body, muſt ap⯑pear in the form of a fluid; and it is this ſluid to which I have given the appellation of the fe⯑male ſemen.

This fluid is not inert, as Ariſtotle pretends, but prolific, and equally eſſential to generation as the ſemen of the male. It contains particles diſtinctive of the female ſex, as that of the other ſex contains particles proper for the conſtitution of male organs; and each of them contain all the other organic particles which may be re⯑garded as common to both ſexes: And hence, from a mixture of the two, the ſon may reſem⯑ble his mother, and the daughter her father. Hippocrates maintains, that the ſemen conſiſts of two fluids, one ſtrong, which produces males, and the other weak, which produces females. But, as the ſeminal fluid is extracted from every part of the body, it is impoſſible to conceive how the body of a female ſhould produce particles proper for the formation of male organs.

This liquor muſt enter, by ſome way or other, into the uterus of viviparous animals; and, in oviparous animals, it muſt be abſorbed by the [242] eggs, which may be regarded as portable ma⯑trixes. Each of theſe matrixes, or eggs, con⯑tains a ſmall drop of the female fluid, in that part which is called the cicatrice. This prolific drop, when the female has had no communica⯑tion with the male, aſſumes, as Malpighius ob⯑ſerves, the form of a mole or inorganic maſs; but, when it is penetrated by the ſemen of the male, it produces a foetus, which is nouriſhed and brought to perfection by the juices of the egg.

Eggs, therefore, inſtead of being common to all females, are only inſtruments employed by nature for ſupplying the place of uteri in thoſe animals which are deprived of this organ. Inſtead of being active and eſſential to the firſt impreg⯑nation, eggs are only paſſive or accidental parts, deſtined for the nouriſhment of the foetus al⯑ready formed in a particular part of this matrix by the mixture of the male and female ſemen. Inſtead of exiſting from the creation, and each including within itſelf an infinity of males and females, eggs, on the contrary, are bodies com⯑poſed of a ſuperfluous part of the food, which is more groſs, and leſs organic, than that of which the ſeminal fluid conſiſts. The egg, in oviparous females, anſwers the ſame purpoſes as the uterus and menſtrual flux in the vivipa⯑rous.

To evince that eggs are only deſtined by na⯑ture to ſupply the place of an uterus in ſuch ani⯑mals [243] as are deprived of this organ, we have on⯑ly to conſider, that females produce eggs inde⯑pendent of the male. The uterus, in viviparous animals, is a part peculiar to the female ſex; in the ſame manner, female fowls, who want this organ, have the defect amply ſupplied by the ſucceſſive production of eggs, which neceſ⯑ſarily exiſt in theſe females, independent of all communication with the male. To pretend that the foetus pre-exiſted in the egg, and that eggs are contained, ad infinitum, within each other, is equally ridiculous as to maintain that the foetus pre-exiſted in the uterus, and that the uterus of the firſt female contained all the uteri that ever were or will be produced.

Anatomiſts have applied the term egg to things [...] very oppoſite natures. Harvey, in his [...], Omnia ex ovo, by the word egg, when applied to oviparous animals, means only the bag which includes the foetus and all its appendages. He imagined that he perceived the formation of this egg or bag immediately after the junction of the male and female. But this egg proceeded not from the ovarium of the female: He even aſſerts, that he could not di⯑ſtinguiſh the ſmalleſt alteration in the ovarium. It is apparent, that there is not here the moſt diſtant analogy to what is commonly underſtood by the word egg, unleſs, perhaps, the figure of the bag might have ſome faint reſemblance to that of an egg. Harvey, though he diſſected [244] many viviparous females, never could perceive any change in their ovaria: He even regards them as glands totally unconnected with the purpoſes of generation^*, though, as we have ſeen, they undergo very conſiderable changes. This able anatomiſt was deceived by the ſmall⯑neſs of the glandular bodies in animals of the deer kind, to which his reſearches were princi⯑pally confined. Conradus Peyerus, who made many obſervations on the teſticles of female deers, remarks, 'Exigui quidem ſunt damarum teſticuli, ſed poſt coitum foecundum, in alter⯑utro eorum, papilla, ſive tuberculum fibroſum, ſemper ſuccreſcit; ſcrofis autem praegnanti⯑bus tanta accidit teſticulorum mutatio, ut me⯑diocrem quoque attentionem fugere nequeat^†.' This author aſcribes, with propriety, the reaſon why Harvey obſerved no changes in the teſticles of the deer, to their ſmallneſs. But he is wrong when he tells us, that the changes he had re⯑marked, and which had eſcaped Harvey, never happened but after impregnation.

Harvey was deceived in ſeveral other eſſential articles. He inſiſts, that the ſemen never enters into the uterus, and that it is impoſſible for it to find admittance; and yet Verheyen found a great quantity of male ſemen in the uterus of a cow, which he diſſected ſix hours after copula⯑tion ^‡. The celebrated Ruyſch informs us, that, [245] in diſſecting a woman who had been killed im⯑mediately after the act of adultery, he found a conſiderable quantity of male ſemen, not only in the uterus, but in the Fallopian tubes^*. Valiſ⯑nieri likewiſe aſſures us, that Fallopius, and other anatomiſts, had diſcovered male ſemen in the u⯑teri of ſeveral women. This point, therefore, though denied by Harvey, is eſtabliſhed by the poſitive teſtimony of ſeveral able anatomiſts, and particularly by Leeuwenhoek, who found male ſemen in the uteri of many different ſpecies of females.

Harvey mentions an abortion, in the ſecond month, as large as a pigeon's egg, without any appearance of a foetus. In this alſo he muſt have been deceived; for Ruyſch, and ſeveral o⯑ther anatomiſts, maintain, that the foetus is di⯑ſtinguiſhable by the naked eye, even in the firſt month of pregnancy. In the Hiſtory of the French Academy, we have an account of a foe⯑tus compleatly formed on the twenty-firſt day after impregnation. If, to theſe authorities, we add that of Malpighius, who diſtinguiſhed the chick in the cicatrice immediately after the egg iſſued from the body of the hen, we cannot heſitate in pronouncing that the foetus is form⯑ed immediately after copulation; and, conſe⯑quently, no credit is due to what Harvey ſays concerning the increaſe of the parts by juxta-po⯑ſition; ſince theſe parts exiſt from the beginning, [246] and gradually expand till the foetus be perfectly mature.

Graaf differs widely from Harvey in his ac⯑ceptation of the word egg. He maintains, that the female teſticles are real ovaria, and contain eggs ſimilar to thoſe of oviparous animals, only that they are much ſmaller, never fall out of the body, nor detach themſelves till after impregna⯑tion, when they deſcend from the ovarium into the uterus. The experiments of Graaf have contributed more to the belief of the exiſtence of eggs, than thoſe of any other anatomiſt. They are, notwithſtanding, totally void of foundation; for this celebrated author, in the firſt place, mi⯑ſtakes the veſicles of the ovarium for eggs, though they are inſeparable from the ovarium, form a part of its ſubſtance, and are filled with a ſpecies of lymph. 2. He is ſtill more deceived, when he informs us, that the glandular bodies are on⯑ly the coverings of theſe eggs or veſicles; for it is certain, from the obſervations of Malpighius and of Valiſnieri, and from my own experi⯑ments, that the glandular bodies contain no ve⯑ſicles. 3. He is wrong in maintaining that the glandular bodies never appear till after impreg⯑nation. On the contrary, theſe bodies are uni⯑formly found in all females, after the age of puberty. 4. He errs in ſuppoſing that the glo⯑bules which he ſaw in the uterus, and which con⯑tained the foetuſes, were the very veſicles that had deſcended from the ovarium into the uterus, [247] and that, he remarks, had become ten times ſmaller than when they were in the ovarium. This circumſtance alone of their diminiſhed ſize ſhould have convinced him of his miſtake. 5. He is not leſs unfortunate in maintaining that the glandular bodies are only the coverings of the fecundated eggs, and that the number of co⯑verings or empty follicles always correſpond to the number of foetuſes. This aſſertion is the reverſe of truth; for, on the teſticles of all fe⯑males, we uniformly find a greater number of glandular bodies, or cicatrices, than of foetuſes actually produced; and they even appear in thoſe which never brought forth. To this we may add, that neither he, Verheyen, nor any other perſon, ever ſaw the egg in this pretended co⯑vering, or in its follicle, though they have thought proper to reſt their ſyſtem upon that ſuppoſi⯑tion.

Malpighius, who diſtinguiſhed the growth of the glandular bodies in the female teſticle, was deceived when he imagined that he once or twice diſcovered the egg in their cavities; for this ca⯑vity contains only a fluid; beſides, after number⯑leſs experiments, no man has ever been able to diſcover any thing that had the moſt diſtant re⯑ſemblance to an egg.

Valiſnieri, who is never deceived with regard to facts, is wrong in maintaining that the egg muſt exiſt in the glandular body, though neither [246] [...] [247] [...] [248] he, nor any man elſe, was ever able to diſcover it.

Let us now attend to what may be eſteemed the real diſcoveries of theſe anatomiſts. Graaf was the firſt who diſcovered that the teſticles of females ſuffered any change; and he was right in maintaining that they were parts eſſentially neceſſary to generation. Malpighius demonſtra⯑ted, that the glandular bodies gradually grew to maturity, and that, immediately after, they were obliterated, and left behind them only a ſlight cicatrice. Valiſnieri illuſtrated this ſubject ſtill farther. He diſcovered that theſe glandular bodies were found in the teſticles of all females; that they were conſiderably augmented in the ſeaſon of love; that they increaſed at the ex⯑pence of the lymphatic veſicles of the teſticle; and that, during the time of their maturity, they were hollow and full of liquor. Theſe are all the truths we have learned concerning the pre⯑tended ovaria and eggs of viviparous animals: What concluſions are we to draw from them? Two things appear to be evident: The one, that no eggs exiſt in the teſticles of females; the o⯑ther, that there is a fluid both in the veſicles of the teſticle, and in the cavity of the glandular bodies; and we have demonſtrated, in the pre⯑ceding experiments, that this laſt fluid is the true female ſemen, becauſe it contains, like that of the male, ſpermatic animals, or rather organic par⯑ticles in motion.

[249] The ſeminal fluid of females, therefore, be⯑ing thus fully aſcertained, after what has been ſaid, we muſt be ſatisfied that the ſeminal fluid in general is the ſuperfluous organic part of our food, which is tranſmitted from all parts of the body to the teſticles and ſeminal veſſels of males, and to the teſticles and glandular bodies of fe⯑males. This liquor, which iſſues through the nipples of the glandular bodies, perpetually be⯑dews the Fallopian tubes, and may caſily find admiſſion into them, either by abſorption, or by the ſmall aperture at their extremity, and in this manner may deſcend into the uterus. But, on the ſuppoſition of the exiſtence of eggs, which are ten or twenty times larger than the aperture of the tubes, it is impoſſible to conceive the poſ⯑ſibility of their being tranſmitted to the uterus.

The liquor ſhed by females in the paroxyſm of love, which Graaf ſuppoſes to proceed from lacunae about the neck of the uterus and the o⯑rifice of the urethra, may be a portion of the ſuperfluous fluid that continually diſtills from the glandular bodies upon the Fallopian tubes. But perhaps this liquor may be a ſecretion of a dif⯑ferent kind, and no way connected with gene⯑ration. To decide this queſtion, microſcopic obſervations would be neceſſary; but all expe⯑riments are not permitted even to philoſophers. I am inclined to think, that, in this liquor, the ſame ſpermatic animals, or moving bodies, would be found as appear in the fluid of the glandular [250] bodies. Upon this ſubject, I might quote the authority of an Italian phyſician, who had an opportunity of trying this experiment, which is thus related by Valiſnieri^*: 'Aggiugne il lo⯑dato ſig. Bono d'avergli anco veduti (animali ſpermatici) in queſta linfa o ſiero, diro coſi voluttuoſo, che nel tempo dell' amoroſa zuffa ſcappa dalle femine libidinoſe, ſenza che ſi po⯑teſſe ſoſpettare che foſſero di que' del maſchio,' &c. If the fact be genuine, as I have no rea⯑ſon to doubt, it is certain that this liquor is the ſame with that contained in the glandular bodies, and, of courſe, that it is a real ſeminal fluid, which eſcapes through the lacunae of Graaf, ſi⯑tuated about the neck of the uterus.

Hence we may conclude, that the moſt libidi⯑nous females will be the leaſt fruitful, becauſe they throw out of the body that fluid which ought to remain in the uterus for the formation of the foetus. We likewiſe learn why common proſtitutes ſeldom conceive; and why women in warm climates, who have more ardent deſires than thoſe of colder regions, are leſs fertile. But of this we ſhall afterwards have occaſion to treat.

The ſpermatic moving bodies may be regarded as the firſt aſſemblages of the organic particles which proceed from all parts of the body; and, when a great number of them unite, they become perceptible by means of the microſcope. But, when the number united is ſmall, the body they form is too minute to be viſible, and no motion will appear in the ſeminal fluid, a caſe which not unfrequently happens. But a long train of ſuc⯑ceſſive experiments would be neceſſary to aſcer⯑tain the cauſes of the different ſtates in which this fluid appears.

Of one thing I am certain, from repeated tri⯑als, that a ſeminal liquor, though no motion can be perceived when it is firſt taken from the bo⯑dy, after being three or four days infuſed in water, produces as great a number of organic moving particles, as another ſemen, treated in the ſame manner, which at firſt contained vaſt multitudes. Theſe moving bodies appear like⯑wiſe in infuſions of the blood, of the chyle, of the fleſh, and even of the urine, as well as in infuſions of all parts of vegetables; and thoſe which appear in all theſe different ſubſtances, ſeem to have nothing peculiar to them. They all move and act nearly in the ſame manner. If we will have theſe bodies to be animated, it muſt be al⯑lowed, [252] that they are very imperfect, and ought to be regarded only as the rudiments of animals, or rather as bodies compoſed of particles eſſen⯑tial to the exiſtence of animals. As Nature's productions are uniform, and advance by imper⯑ceptible degrees, there is no improbability in ſuppoſing the exiſtence of organized bodies which properly belong not either to the animal or ve⯑getable kingdoms.

However this matter may ſtand, it is fully a⯑ſcertained, that all animal and vegetable ſub⯑ſtances contain an infinite number of living or⯑ganic particles. Theſe particles ſucceſſively aſ⯑ſume different forms, and different degrees of activity, according to different circumſtances. They are more abundant in the ſeminal fluids of both ſexes, and in the ſeeds of plants, than in a⯑ny other part of the animal or vegetable. There exiſts, therefore, in vegetables and animals, a li⯑ving ſubſtance which is common to them both; and this ſubſtance is the matter neceſſary to their nutrition. The animal is nouriſhed by vegeta⯑ble or animal ſubſtances; and the vegetable is nouriſhed by the ſame ſubſtances in a decompo⯑ſed ſtate. This common nutritive ſubſtance is always alive and active. It produces an animal or a vegetable, whenever it finds an internal mould or matrix accommodated to the one or the other, as has already been explained. But, when this active ſubſtance is collected too abun⯑dantly in a place where it has an opportunity of [253] uniting, it forms, in the animal body, other li⯑ving creatures, as the tape-worm, the aſcarides, the worms ſometimes found in the veins, in the ſinuſes of the brain, in the liver, &c. Animals of this kind owe not their exiſtence to the ge⯑neration of individuals of the ſame ſpecies. It is, therefore, natural to think, that they are produced by an extravaſation of the organic matter, or by an inability in the lacteal veſſels to abſorb the quantity of it preſented to them. But we ſhall afterwards have occaſion to exa⯑mine more in detail the nature of theſe worms, and of other animals which are produced in a ſimilar manner.

When this organic matter, which may be con⯑ſidered as an univerſal ſemen, is aſſembled in great quantities, as in the ſeminal fluids, and in the mucilaginous part of the infuſions of plants, its firſt effect is to vegetate, or rather to produce vegetating beings. Theſe zoophytes ſwell, ex⯑tend, ramify, and then produce globular, oval, and other ſmall bodies of different figures, all of which enjoy a ſpecies of animal life; they have a progreſſive motion, which is ſometimes very rapid, and ſometimes more ſlow. The globules themſelves decompoſe, change their figure, and become ſmaller; and, in proportion as they di⯑miniſh in ſize, the rapidity of their motion in⯑creaſes.

I have ſometimes imagined, that the venom of the viper, and even the poiſon of enraged ani⯑mals, [254] might proceed from this active matter be⯑ing too much exalted. But I have not yet had leiſure for experiments of this kind, nor for a⯑ſcertaining the nature of different drugs. All I can ſay at preſent is, that infuſions of the moſt active drugs abound with moving bodies, and that they appear ſooner in them than in other ſubſtances.

Almoſt all microſcopic animals are of the ſame nature with the moving bodies in the ſeminal fluids, and in infuſions of animal and vegetable ſubſtances. The eels in paſte, in vinegar, &c. are all of the ſame nature, and derived from the ſame origin. But the proofs and illuſtrations relative to this ſubject, we ſhall reſerve till we give the particular hiſtory of microſcopic a⯑nimals.

CHAP. IX.
Varieties in the Generation of Animals.

THE nutrition and the reproduction of a⯑nimals and of vegetables, are thus effected by the ſame matter. It is a ſubſtance univerſal⯑ly prolific, and compoſed of organic particles, the union of which gives riſe to all organized bodies. Nature always works on the ſame ſtock, and this ſtock is inexhauſtible. But the means ſhe employs to give it value are various; and theſe general varieties and affinities merit the at⯑tention of philoſophers, becauſe from them we are enabled to account for particular exceptions to the common plan of her operations.

In general, large animals are leſs prolific than ſmall ones. The whale, the elephant, the rhi⯑noceros, the horſe, man, &c. produce but one, and very rarely two, at a birth. But ſmall ani⯑mals, as rats, herrings, and inſects, produce a great number. Does this difference proceed from the greater quantity of nouriſhment ne⯑ceſſary to ſupport the large animals than the ſmall, and from the former having a leſs pro⯑portional quantity of ſuperfluous nutritive par⯑ticles, [256] capable of being converted into ſemen, than the former? It is certain, that the ſmall animals eat more, in proportion to their bulk, than the large. But it is likewiſe probable, that the prodigious increaſe of the ſmaller animals, as bees, flies, and other inſects, may be owing to the extreme fineneſs and delicacy of their or⯑gans and members, by which they are enabled to ſelect the moſt ſubſtantial and moſt organic parts of the animals and vegetables, from which they extract their nouriſhment. A bee, which lives upon the pureſt and moſt refined parts of flowers, receives from its food a greater propor⯑tional quantity of organic particles than a horſe, who feeds upon hay, ſtraw, and the groſſer parts of vegetables. The horſe, accordingly, produ⯑ces but one at a time, while the bee produces many thouſands. The oviparous animals are, in general, ſmaller than the viviparous, and they are likewiſe much more prolific. The long time that the foetus remains in the uterus of vivipa⯑rous animals, is another obſtacle to multiplica⯑tion: During geſtation, and the ſuckling of the young, no new generation can take place. But the oviparous animals, which produce, at the ſame time, both uterus and foetus, and throw them out of the body, are almoſt perpetually in a condition to reproduce; and it is well known, that, if a hen be prevented from ſitting, and be fully fed, the number of her eggs may be great⯑ly increaſed. If hens lay not while they brood, [257] it is becauſe they ceaſe to eat; and for this pur⯑poſe they leave not their neſts but once a day, and even then for a very ſhort time, leſt their eggs ſhould be injured by the cold. During this operation, they take not above one tenth part of their ordinary nouriſhment.

Animals which produce but one at a birth, ac⯑quire nearly their full growth before they are fit for generating. But thoſe which produce many, generate before they are half grown. Man, the horſe, the aſs, the ſheep, are incapable of gene⯑ration till after they have nearly acquired the greateſt part of their growth. It is the ſame with pigeons and other birds that lay but a ſmall number of eggs: But thoſe that are more pro⯑lific, as cocks and hens, fiſhes, &c. begin to ge⯑nerate much ſooner. A cock is capable of this operation at the age of three months, when he is not above one third of his full ſize. A fiſh, which, in 20 years, will weigh 30 pounds, ge⯑nerates the firſt or ſecond year, when it weighs not, perhaps, more than half a pound. But ex⯑periments are ſtill wanting to aſcertain the growth and duration of fiſhes: Their age may be diſ⯑covered by examining with a microſcope the an⯑nual rings or ſtrata of which their ſcales are compoſed. But we are ignorant how far this may extend. I have ſeen, in the Count de Maurepas' ponds, carps which were well atteſted to be at leaſt 150 years old, and they appeared to be equally active and lively as common carps. [258] I will not ſay with Leeuwenhoek, that fiſhes are immortal, or, at leaſt, that they cannot die of old age. Every thing, in time, muſt periſh. Whatever has an origin, a birth, or commence⯑ment, muſt arrive at a termination or death. But fiſhes, by living in a uniform element, and being ſheltered from the injurious viciſſitudes of the air, muſt continue longer in the ſame ſtate than other animals, eſpecially if theſe viciſſitudes, as Bacon remarks, be the chief cauſes of the deſtruction of animated beings. But the prin⯑cipal cauſe of the longevity of fiſhes is, that their bones are ſofter than thoſe of other animals, and do not perceptibly harden with age. The bones of fiſhes lengthen, and turn thick without ac⯑quiring more ſolidity. But the denſity of the bones of other animals continually increaſes; and, when their interſtices are completely filled and obſtructed, the circulation of their fluids ceaſes, and death enſues. But, in the bones of fiſhes, this augmentation of ſolidity, which is the natural cauſe of death, proceeds in ſuch an im⯑perceptible manner, that they muſt live very long before they can feel any of the effects of old age.

All quadrupeds covered with hair are vivipa⯑rous, and thoſe covered with ſcales are ovipa⯑rous. The cloſe texture of the ſhells or ſcales of oviparous animals prevents them from loſing ſo much matter by tranſpiration, as makes its way through the porous ſkins of the viviparous. [259] May not this retention of ſuperfluous nouriſh⯑ment, which cannot eſcape by tranſpiration, be one reaſon of the extraordinary fertility of theſe animals, and of their being able to ſubſiſt a long time without food? All birds and flying inſects are oviparous, except ſome ſpecies of flies which produce their young alive^*. Theſe have no wings immediately after their birth; but they gradually ſhoot out as the animal advances in growth; and they are not in a condition to be uſed till it acquires full maturity. All ſhell⯑fiſhes are viviparous; and likewiſe thoſe reptiles that have no feet, as ſnakes and ſerpents; they change their ſkins, which are compoſed of ſmall ſcales. The viper is but a ſlight objection to this general rule; for it is not properly vivipa⯑rous. It firſt produces eggs, from which the young are hatched: This operation is indeed carried on and compleated in the body of the mother; and, in place of laying the eggs, like other oviparous animals, the viper hatches them within the body. The ſalamander, in which, as Maupertuis remarks^†, both eggs and young are found at the ſame time, is a ſimilar exception in oviparous quadrupeds.

Moſt animals are multiplied and perpetuated by copulation. But many animals, as the great⯑eſt number of birds, propagate rather by a kind of compreſſion, than a proper copulation. Some [260] birds, indeed, as the oſtrich, the male duck, &c. have conſiderable members, and propagate by a real intromiſſion. Male fiſhes approach the fe⯑males in the ſeaſon of ſpawning. They ſeem to rub their bellies againſt each other; for the male often turns on his back to meet the belly of the female. But no actual copulation takes place. The part neceſſary for this operation does not exiſt; and the males only approach the females for the purpoſe of ſhedding the liquor of their milts upon the eggs, which at that ſea⯑ſon drop from the females. The male ſeems to be more attached to the eggs than to the female; for, when ſhe ceaſes to throw out the eggs, the male inſtantly abandons her, and follows, with ardor, the eggs which are carried down by the ſtream, or diſperſed by the winds. He paſſes and repaſſes a thouſand times over every place where he finds eggs. It is not, ſurely, for the love of the mother that he makes all theſe move⯑ments: He cannot even be ſuppoſed to know her; for he has been often ſeen ſhedding his ſemen promiſcuouſly on all the eggs that came in his way, without having ever met with the female to which they belonged.

Thus ſome animals are diſtinguiſhed by ſexes, and endowed with members proper for copula⯑tion. There are others which likewiſe have ſexes, but want the neceſſary members. Others, as ſnails, have both members and ſexes in each individual. Others, as the vine-fretters, have [261] no ſexes, are equally fathers or mothers, and pro⯑duce of themſelves without copulation. Though they ſeem to copulate at pleaſure, we are unable to diſcover the uſe of their junction, or whether it be really a ſexual embrace; unleſs we ſhould ſuppoſe Nature to have endowed this ſmall in⯑ſect with generative faculties ſuperior to thoſe of any other ſpecies of animals, and to have be⯑ſtowed on every individual not only the power of reproduction, but likewiſe the power of mul⯑tiplying by ſexual communications.

But, whatever varieties take place in the ge⯑neration of different ſpecies of animals, Nature prepares the body for it by a new production, which, whether it be external or internal, always precedes generation: Immediately before the ſeaſon of impregnation, the ovaria of oviparous animals, and the teſticles of the females of the viviparous, undergo a conſiderable change. The oviparous animals produce eggs, which gradual⯑ly increaſe in ſize, till they quit the ovarium and fall into the canal of the uterus, where they receive their white, their membranes, and their ſhell. This production marks the fecundity of the female, and without which generation could not be effected. In viviparous females, in the ſame manner, one or more glandular bodies ap⯑pear upon the teſticles, which gradually grow under the membranes that covers them. Theſe glandular bodies increaſe, and pierce, or rather elevate the membrane of the teſticle; and, when [262] they arrive at maturity, a fiſſure, or ſeveral little holes, appear at their extremities, through which the ſeminal fluid eſcapes, and falls into the ute⯑rus. Theſe glandular bodies are new produc⯑tions, which always precede generation, and without which it could not be effected.

Males undergo a ſimilar change before they are fit for the purpoſes of generating. In the oviparous animals, the ſeminal reſervoirs are filled, and ſometimes the reſervoirs themſelves are annually renewed. The milts of ſome fiſhes, and particularly of the calmar, are renewed every year. The teſticles of birds, immediately be⯑fore the ſeaſon of their amours, ſwell to an e⯑normous degree. The teſticles of the males of viviparous animals, eſpecially of thoſe which have ſeaſons, likewiſe ſwell conſiderably; and, in ge⯑neral, the genitals of every ſpecies ſuffer an e⯑rection, which, though it be external and caſu⯑al, may be regarded as a new production that neceſſarily precedes the faculty of generating.

Thus, in all animals, whether male or female, generation is always preceded by new produc⯑tions; and, when there is properly no new pro⯑ductions, ſome of the parts ſwell and extend to a remarkable degree. In ſome animals, not on⯑ly a new production appears, but their whole bodies are renewed, before generation can be effected; as happens in the ſurpriſing metamor⯑phoſis of inſects, which ſeems to be intended for no other purpoſe than to enable theſe ani⯑mals [263] to propagate their ſpecies; for their bodies are full grown before they are transformed. The inſect, immediately before its transforma⯑tion, ceaſes to take nouriſhment; and it has no organs proper for generation, no means of con⯑verting the nutritive particles, with which it a⯑bounds more than any other ſpecies of animals, into eggs, or a ſeminal fluid. Hence the whole of this great ſurplus of nutritive particles at firſt unites and moulds itſelf into a form nearly re⯑ſembling that of the original animal. The ca⯑terpillar becomes a butterfly, becauſe, having no organs of generation, no reſervoirs for contain⯑ing the ſuperfluous nutritive particles, and, con⯑ſequently, being incapable of producing minute organic bodies ſimilar to the animal itſelf, the organic nutritive particles, which are always ac⯑tive, aſſume, by their union, the form of a but⯑terfly, partly reſembling that of a caterpillar, both internally and externally, excepting that the parts of generation are unfolded, and ren⯑dered capable of receiving and tranſmitting the nutritive organic particles which form the eggs, and individuals peculiar to the ſpecies. The individuals produced by the butterfly ought not to be butterflies, but caterpillars; becauſe it was the caterpillar that received the nouriſhment, and becauſe the organic particles of this nouriſh⯑ment muſt therefore be aſſimilated into the form of a caterpillar, and not of a butterfly, which is only an occaſional production of the ſuperfluous [264] nouriſhment that precedes the real production of animals of this ſpecies, and a method employed by Nature to accompliſh the important purpoſes of generation, ſimilar to the production of glan⯑dular bodies, and of milts, in other animals.

When the ſuperabundant quantity of organic nutritive particles is not great, as in man, and moſt large animals, generation does not take place till the growth of the body is nearly com⯑pleted; and even their prolific powers are limit⯑ed to a ſmall number of young: But, when theſe particles are more abundant, as in birds, and o⯑viparous fiſhes, generation is effected before the animal be fully grown, and the production of individuals is very numerous. When the quan⯑tity of organic nutritive particles is ſtill greater, as in inſects, it firſt produces a large organized body, retaining the internal and eſſential conſti⯑tution of the animal, but differing in ſeveral parts, as the butterfly differs from the caterpil⯑lar; and then it quickly generates an amazing number of young, ſimilar to the animal that firſt prepared the organic nouriſhment from which they derived their origin. Laſtly, when the quantity of ſuperfluous nouriſhment is very great, and when the animal, at the ſame time, poſſeſſes the organs neceſſary to generation, as in the vine-fretters, it firſt confers on each indi⯑vidual the power of generating, and then a transformation, like that which other inſects undergo: The vine-fretter becomes a flie; but [265] it can produce nothing, becauſe it is only the reſidue of the organic particles that had not been employed in the production of the young.

Almoſt all animals, man excepted, have cer⯑tain annual ſeaſons appropriated to the purpoſes of generation. To birds, ſpring is the ſeaſon of love: Carps, and ſeveral other fiſhes, ſpawn in June and Auguſt. Pikes, and ſome other fiſhes, ſpawn in the ſpring. Cats have three ſeaſons annually, in the months of January, May, and September. The roe-deer rut in December, wolfs and foxes in January, horſes in ſummer, ſtags in September and October; and almoſt all inſects generate only during the autumn. Some animals, as the inſects, are totally exhauſted by generation, and die ſoon after it. Others, though they die not, become feeble, are much ema⯑ciated, and require a conſiderable time to re⯑pair the great waſte of their organic ſubſtance. Others are leſs affected, and are capable of frequently renewing their amours; laſtly, man is very little affected, or, rather, he quick⯑ly repairs the loſs, and therefore is, at all times, in a condition for propagating. All theſe varieties ſolely depend on the particular con⯑ſtruction of the animal organs. The limits fix⯑ed by Nature upon the modes of exiſting are e⯑qually conſpicuous in the manner of taking and digeſting the food, in the means employed for retaining or throwing it out of the body, and in the inſtruments by which the organic particles [266] neceſſary to reproduction are extracted. And, upon the whole, it is apparent, that every thing exiſts which can exiſt.

The times of the geſtation of females are e⯑qually various: Some, as mares, carry their young from eleven to twelve months; others, as women, cows, and hinds, carry their young nine months; others, as foxes and wolves, carry five months; bitches carry nine weeks, cats ſix weeks, and rabbits thirty-one days. Moſt birds are hatched in twenty-one days; though ſome of them, as the thiſtle-finches, hatch in thirteen or fourteen days. Here the variety is equally great as in every other part of the oeconomy of ani⯑mals: The largeſt animals produce fewer young, and carry them longeſt; which confirms the doc⯑trine, that the quantity of organic nouriſhment is proportionally leſs in large than in ſmall ani⯑mals; for the foetus derives its growth and the expanſion of its parts from the ſuperfluous nou⯑riſhment of the mother; and, as this growth requires longer time in large than in ſmall animals, it is a proof that the quantity of or⯑ganic particles is not ſo great in the former as in the latter.

Animals, therefore, are much diverſiſied as to the time and manner of geſtation, of engender⯑ing, and of producing; and this variety origi⯑nates from the very cauſes of generation. For, though the organic matter, which is common to every thing that lives or vegetates, be the gene⯑ral [267] principle of reproduction, the manner of its union, and the combinations it forms, muſt be infinitely varied, that the whole may become the ſources of new productions. My experiments clearly demonſtrate, that there are no pre-exiſt⯑ing germs, and that the generation of animals and vegetables is not univocal. There are, per⯑haps, as many beings, which either live or vege⯑tate, produced by a fortuitous aſſemblage of or⯑ganic particles, as by a conſtant and ſucceſſive generation. It is to ſuch productions that we ought to apply the axiom of the antients, Cor⯑ruptio unius, generatio alterius. The corruption and reſolution of animals and vegetables pro⯑duce an infinite variety of organized bodies: Some of them, as thoſe of the calmar, are only a kind of machines, which, though exceedingly ſimple, are very active. Others, as the ſper⯑matic animalcules, ſeem to imitate the move⯑ments of animals. Others reſemble vegetables in their manner of growth and expanſion. There are others, as thoſe of blighted wheat, which, at pleaſure, can be made alternately to live or to die; and it is difficult to know to what they ſhould be compared. There are ſtill others, and in great numbers, which are at firſt a kind of animals, then become a ſpecies of vegetables, and again return alternately to their vegetable ſtate. The more we examine this ſpecies of organized bo⯑dies, we ſhall probably diſcover greater and more ſingular varieties of them, in proportion as they [268] are farther removed from our obſervation, and from the ſtructure of other animals with which we are already acquainted.

For example, blighted corn, which is effected by an alteration or reſolution of the organic ſubſtance of the grain, is compoſed of multitudes of ſmall organized bodies reſembling eels. When infuſed in water for ten or twelve hours, we diſcover them to have a diſtinct wreathing, and a ſmall degree of progreſſive motion. They ceaſe to move as ſoon as the water fails them; and their motion commences upon the addition of freſh water: This alternate death and reviviſ⯑cence may be repeated for months, and even for years; ſo that theſe ſmall machines may be made to act as long and as often as we pleaſe, without deſtroying or diminiſhing their force. They are a ſpecies of machines, which begin to act whenever they are immerſed in a fluid. Theſe fi⯑laments ſometimes open like the filaments of the ſemen, and produce moving globules: We may, therefore, conclude them to be of the ſame nature, excepting that they are more fixed and ſolid.

The eels in paſte have no other origin than the union of the organic particles of the moſt eſſential parts of the grain. The firſt eels which appear are certainly not produced by other eels; but, though they are not propagated themſelves, they fail not to engender other living eels. By cutting them with the point of a lancet, we diſ⯑cover [269] ſmaller eels iſſuing in great numbers out of their bodies. The body of this animal ſeems to be only a ſheath or ſac containing a multitude of ſmaller animals, which perhaps are other ſheaths of the ſame kind, in which the organic matter is aſſimilated into the form of eels.

A great number of experiments would ſtill be neceſſary to diſtinguiſh theſe animals, which are ſo ſingular and ſo little underſtood, into claſſes and genera. Some of them may be regarded as real zoophytes, which enjoy a kind of vege⯑tation, and which, at the ſame time, ſeem to wreath and move like animals. Others appear, at firſt, to be animals, and then join and form a ſpecies of vegetables. A ſmall attention to the reſolu⯑tion of a ſingle grain of corn will elucidate, at leaſt in part, what I have ſaid on this ſubject. I might add other examples; but thoſe I have gi⯑ven were only produced for the purpoſe of exhi⯑biting the varieties of generation. There are unqueſtionably ſeveral organized bodies which we conſider as real animals, but which are not engen⯑dered by others of the ſame ſpecies. Some of them are only a kind of machines; and ſome of theſe machines have a certain limited effect, and act only for a certain time, as the machines in the milt of the calmar; others may be made to act as long and as often as we pleaſe, as thoſe of blighted grain. There are vegetables which produce animated bodies, as the filaments of the human ſemen, from which active globules iſſue [270] and move by their own powers. In the corrup⯑tion, the fermentation, or rather in the reſolu⯑tion of animal or vegetable ſubſtances, we find real animals capable of propagating their ſpecies, though they were not themſelves produced in this manner. Theſe varieties are, perhaps, more extenſive than we imagine. Though it be right to generalize our ideas, to aſſemble the effects of Nature under one point of view, and to claſs her productions; yet numberleſs ſhades, and even degrees, in the great ſcale of being, will always eſcape our obſervation.

CHAP. X.
Of the Formation of the Foetus.

FROM the experiments of Verheyen, who found the ſemen of the bull in the uterus of a cow, and from thoſe of Ruyſch, Fallopius, and Leeuwenhoek, who diſcovered male ſemen in the uteri of women and many other animals, it ſeems to be a point fully aſcertained, that the ſemen of the male enters into the uterus of the female. It is probable, that, during the time of coition, the orifice of the uterus opens for the reception of the ſeminal fluid: But, though this ſhould not happen, the active and prolific part of the ſemen may penetrate the membranes and ſubſtance of the uterus itſelf; for, as the ſemi⯑nal liquor, as formerly remarked, is almoſt en⯑tirely compoſed of organic particles, which are very active, and extremely minute, they may paſs with the utmoſt facility through the mem⯑branes and ſubſtance of the uterus.

What proves that the active part of this fluid may paſs through the pores and ſubſtance of the uterus, is the ſudden change it undergoes immediately after conception. The menſes are [272] obſtructed, the uterus becomes flaccid, ſwells, and appears to be inflated. All theſe changes muſt be effected by an active external cauſe, by the penetration of part of the ſeminal fluid into the ſubſtance of the uterus. This penetration is not confined to the ſurface; it extends through all the veſſels and parts of which the uterus is compoſed, like that penetration by which nu⯑trition, and the expanſion of the body is pro⯑duced.

We ſhall the more eaſily believe this to be the caſe, when we reflect, that, during the time of geſtation, the uterus not only augments in ſize, but even in its quantity of matter, and that it poſſeſſes a ſpecies of life, or rather of vegetation, which continues till the child be delivered. If the uterus were only a ſac, a reſervoir for re⯑ceiving the ſemen and retaining the foetus, it would extend and diminiſh in thickneſs, in pro⯑portion as the foetus grew larger. But the aug⯑mentation of the uterus is not a ſimple exten⯑ſion or dilatation of its parts. It not only extends as the foetus enlarges, but it acquires, at the ſame time, an additional thickneſs and ſolidity; or, in other words, both its ſize and quantity of matter are greatly increaſed. This augmenta⯑tion is a real growth or increaſe of ſubſtance, ſimilar to the expanſion of the body in young animals, which could not be effected but by an intimate penetration of organic particles analo⯑gous to the ſubſtance of thoſe parts. As this [273] expanſion of the uterus never happens but after impregnation, the ſeminal liquor muſt be the cauſe by which it is produced; for the uterus is conſiderably augmented before the foetus has acquired bulk enough to dilate it by preſſing a⯑gainſt its internal ſurface.

It appears, from my own experiments, to be equally certain, that the female has a ſeminal fluid, which begins to be formed in the teſticles, and is brought to maturity in the glandular bo⯑dies. This fluid perpetually diſtills through the ſmall apertures in the extremities of theſe bo⯑dies, and, like that of the male, enters the uterus by two different ways, either through the aper⯑tures at the extremities of the horns of the ute⯑rus, or by piercing through the ſubſtance of the uterus itſelf.

Theſe two ſeminal fluids are extracts from all parts of the body; and a mixture of them is all that is neceſſary for the formation of a certain number of males and females. The more any animal abounds in this ſeminal fluid, or the more it abounds in organic particles, the number of young is the greater, as may be remarked in the ſmaller animals; and the number of young di⯑miniſhes in proportion as the organic particles are leſs abundant, as is the caſe with the larger animals.

But, before taking any farther notice of other animals, we ſhall examine with attention the formation of the human foetus. In mankind, as well as in the larger ſpecies of animals, the [274] quantity of organic particles in the male and fe⯑male ſemen is not great, and, accordingly, they very ſeldom produce above one foetus at a time. This foetus is either a male or a female, accor⯑ding as the number of organic particles predo⯑minates in the male or in the female fluid; and the child reſembles the father or the mother moſt, according to the proportional quantities of male or female organic particles in the mixture of the two liquors.

I conceive, therefore, that the ſeminal fluids, both of the male and of the female, are equally active, and equally neceſſary for the purpoſes of propagation: And this, I think, is fully proven by my experiments; for I found in both fluids the ſame moving bodies; I diſcovered that the male fluid enters into the uterus, where it meets with the fluid of the female; that theſe two fluids are perfectly analogous; and that they are compoſed of parts not only ſimilar in their form, but in their action and movements^*. Now, I imagine, that, by the mixture of the two fluids, the activity of the organic particles proper to each is ſtopped; that the action of the one coun⯑t [...]alances the action of the other; that each or⯑g [...]ic particle, by ceaſing to move, remains fixed in the place which correſponds to its nature; and that this place can be no other than that which it formerly occupied in the body of the animal from which it was extracted. Thus all the orga⯑nic particles which were detached from the head [275] of the animal, will arrange themſelves in a ſi⯑milar order in the head of the foetus. Thoſe which proceeded from the back bone, will diſpoſe themſelves in an order correſponding to the ſtructure and poſition of the vertebrae. In the ſame manner, the organic particles which had been detached from any part of the body, will naturally aſſume the ſame poſition, and arrange themſelves in the ſame order that they obſerved before they were ſeparated from that part. Of courſe, theſe particles will neceſſarily form a ſmall organized body, entirely ſimilar to the ani⯑mal from which they originally proceeded.

It is worthy of remark, that this mixture of the organic particles of both ſexes contains par⯑ticles that are ſimilar, and particles that are diſ⯑ſimilar. The ſimilar particles are thoſe which have been detached from all the parts that are common to the two ſexes. The diſſimilar particles are thoſe which have been ſeparated from the parts that diſtinguiſh the two ſexes. In this mixture, therefore, there is a double portion of particles de⯑ſtined for the formation of the head, the heart, and ſuch parts as are common to both ſexes; while there are no more than what are neceſſary for the production of the ſexual parts. Now, the ſimilar particles may act upon each other without pro⯑ducing any diſorder; and they may unite in the ſame manner as if they had proceeded from the ſame body. But the diſſimilar parts cannot act upon each other, nor form any intimate union, becauſe they have no analogy or relation: Hence [276] theſe particles will preſerve their original nature without any mixture, will firſt fix their poſition, without the neceſſity of being penetrated by o⯑thers. Thus the particles which proceed from the ſexual parts will be firſt fixed, and thoſe that are common to the two ſexes, whether they be⯑long to the male or to the female, will then ſix indiſcriminately, and form an organized body, which, in its ſexual parts, will perfectly reſemble the father, if it be a male, and the mother, if it be a female, but, which, in the other parts, may reſemble either or both.

If what I have advanced be properly under⯑ſtood, we ſhall, perhaps, be able to obviate an objection made to the ſyſtem of Ariſtotle, and which might alſo be urged againſt the doctrine which I am now eſtabliſhing. The queſtion is, Why is not every individual, both male and fe⯑male, endowed with the faculty of producing an animal of its own ſex? I am aware of the diffi⯑culty of ſolving this queſtion, which I have ſlightly mentioned in the fifth chapter, and ſhall now proceed farther to illuſtrate.

From what is delivered in the firſt four chap⯑ters, and from the experiments I have deſcribed, it is apparent, that reproduction is effected by the aſſemblage and union of the organic par⯑ticles, detached from every part of the animal or vegetable body, in one or ſeveral common re⯑ſervoirs; that theſe particles are the ſame which ſerve for the nutrition and expanſion of the [277] body; and that both effects are produced by the ſame matter, and by the ſame laws. I think I have eſtabliſhed this point by ſo many facts and rea⯑ſonings, that it is impoſſible to entertain a doubt concerning its truth. But, I allow, that the queſtion may be put, Why every ſeparate animal and vegetable produces not its own likeneſs, ſince every individual detaches from all its parts, and collects in a common reſervoir, the organic particles neceſſary for the formation of a ſmall organized body? Why is not this organized body formed? and why, in moſt animals, is a mixture of the fluids of both ſexes neceſſary? If I were to reply, that in all the vegetable tribes, in all thoſe animals that multiply by cutting, and in the vine-fretters, which produce without any ſexual commerce, the general intention of Nature ſeems to be, that each individual ſhould multiply its own ſpecies, and that reproduction by the intervention of ſexes is only an exception to this general law. It might, with propriety, be rejoined, that the exception is perhaps more univerſal than the rule itſelf. To maintain that all individuals would have the faculty of re⯑producing, if they were endowed with proper organs, and if they contained the matter neceſ⯑ſary for nouriſhing the embryo, is not removing the difficulty: For, in females, all theſe circum⯑ſtances concur; and yet the influence of the male is indiſpenſible to the production either of a female or of a male foetus.

[278] But, we come nearer a ſolution of the queſ⯑tion, when we maintain, that, though the fluid in the teſticles and ſeminal veſſels of the male contain all the organic particles neceſſary for the formation of a male foetus, yet theſe particles cannot receive any local eſtabliſhment or ar⯑rangement of parts, becauſe a conſtant circula⯑tion of them goes on by means of abſorption, and by the perpetual ſucceſſion of new ſupplies from all parts of the body; and that, as the ſame circulation of the organic particles takes place in the female, neither of them can poſ⯑ſibly multiply without the aſſiſtance of the other; becauſe, in the mixture of the male and female fluids, the different organic particles of which they conſiſt have a greater affinity to each other than they have to the body of the female where the mixture happens. But, though this explication were admitted, why, it may ſtill be aſked, does not the ordinary mode of gene⯑ration correſpond with it? For, upon this ſup⯑poſition, each individual would produce, and, like ſnails, mutually impregnate one another, every individual receiving the organic particles furniſhed by the other, which, without being injured by any other power, would unite ſolely by the affinity between the particles themſelves. If there were no other cauſe by which the or⯑ganic particles could be united, perhaps this mode of generation would be the moſt ſimple. But it is contrary to the analogy of Nature. [279] Few animals are endowed, like ſnails, with both ſexes; and, therefore, if this mode of propa⯑gating were the moſt ſimple, it would be more generally employed by Nature. This ſolution, of courſe, amounts to no more than a gratuitous ſuppoſition, that males produce not, ſolely be⯑cauſe they have not organs proper for contain⯑ing and nouriſhing a foetus.

It may be ſtill farther ſuppoſed, that the acti⯑vity of the organic particles in the ſemen of each individual requires to be counterbalanced by the force or action of thoſe of the other individual, in order to reduce them to a fixed ſtate, or e⯑quilibrium, without which the formation of the foetus cannot be effected; and that the motion of the organic particles of the female cannot be counterbalanced by any other cauſe than a con⯑trary action in the organic particles received from the male. But this anſwer is too general to be void of obſcurity. However, when we attend to all the phaenomena, it may, perhaps, admit of ſome illuſtration. The mixture of the two ſeminal fluids produces not only a male or fe⯑male foetus, but other organized bodies, which are endowed with the faculty of growth or ex⯑panſion. The placenta, the membranes, &c. are produced at the ſame time, if not ſooner, than the foetus. There are, therefore, in the ſeminal fluid of the male or female, or in the mixture of both, organic particles not only ſuit⯑ed to the production of the foetus, but of the [280] placenta and membranes. Since there are no parts either in the male or female bodies from which they could be detached, whence do theſe particles proceed? It muſt be admitted, that the organic ſeminal particles of each ſex being equally active, uniformly produce organized bodies every time that they can ſix themſelves, by their mutual action upon one another; that, of the particles deſtined to produce a male, thoſe peculiar to the male ſex will fix firſt, and form the ſexual parts; that the particles common to both ſexes may afterwards fix themſelves indif⯑ferently, in order to form the reſt of the body; and that the placenta and membranes are pro⯑duced by the exceſs of organic particles which have not been employed in the formation of the foetus. If, as we have ſuppoſed, the foetus be a male, all the organic particles peculiar to the female ſex, which have not been employed, as alſo the ſuperfluous particles of both individuals which have not entered into the compoſition of the foetus, and which cannot be leſs than one half of the whole, remain for the formation of the membranes and placenta. If the foetus be a female, the ſame quantity of ſuperfluous organic particles ſtill remain, and are occupied in forming the placenta and membranes.

But it may be ſaid, that the membranes and placenta, upon this ſuppoſition, ought to become another foetus, which would be a male, if the firſt was a female, and a female, if the firſt was [281] a male; becauſe the firſt foetus conſumed only the organic particles peculiar to the ſex of one individual, and the half of thoſe particles which were common to both ſexes; and, of courſe, the ſexual particles of the other individual, and the other half of the common particles, remain ſtill unexhauſted. To this I reply, that the firſt union of the organic particles prevents a ſecond, at leaſt under the ſame form; and that the foe⯑tus, being firſt formed, exerts an external force which deſtroys the natural arrangement of the other organic particles, and throws them into that order which is neceſſary for the formation of the placenta and membranes.

From the experiments and obſervations for⯑merly made, it is apparent, that all animated be⯑ings contain an amazing quantity of living or⯑ganic particles. The life of an animal or vege⯑table ſeems to be nothing elſe than a reſult of all the particular lives (if the expreſſion be ad⯑miſſible) of each of theſe active particles, whoſe life is primitive, and perhaps indiſtructible. Theſe living particles we have found in every animal and vegetable ſubſtance; and we are certain, that all theſe particles are equally ne⯑ceſſary to the nutrition, and, conſequently, to the reproduction of animals and vegetables. That the union of a certain number of theſe particles, therefore, ſhould produce an animated being, it is not difficult to conceive. As each particle is animated, a whole, or any aſſemblage of them, [282] muſt be endowed with life. Theſe living or⯑ganic particles being common to all animated beings, they are capable of forming particular ſpecies of animals, or of vegetables, according to the peculiar arrangement they aſſume. Now, this arrangement depends entirely on the form of the individuals which furniſh the organic particles. If they are furniſhed by an animal, they arrange themſelves under the form pecu⯑liar to its ſpecies, exactly agreeable to that ar⯑rangement they obſerved when they nouriſhed or expanded the animal itſelf. But, does not [...] ſuppoſe the neceſſity of ſome baſe or centre, round which the particles aſſemble in order to unite and form a foetus? This baſis is furniſhed by the particles which form the ſexual parts. I ſhall illuſtrate this point.

As long as the organic particles of either ſex remain alone, their activity produces no effect, becauſe it is not oppoſed by any reſiſtance or re⯑action from particles of a different kind. But, when the male and female liquors are blended, the particles detached from the ſexual parts be⯑ing of a different kind, ſerve as a baſe to fix the activity of the other particles.

Upon this ſuppoſition, that the organic par⯑ticles which, in the mixture of the two fluids, repreſent the ſexual parts of the male, can alone ſerve as a baſis to the particles which proceed from all parts of the female; and that thoſe proceed⯑ing from all parts of the male can only be fixed [283] by the particles which are detached from the ſexual parts of the female; we may con⯑clude, that the ſexual parts of the male foe⯑tus are formed by the organic particles of the father, and the reſt of its body by the organic particles of the female; and, on the contrary, that the female foetus derives nothing but its ſex from the mother, and the reſt of its body from the father. Boys, therefore, excepting in the parts which diſtinguiſh their ſex, ought to reſemble the mother more than the father, and girls ſhould reſemble the father more than the mother.

Conſidering generation by ſexes under this light, we ſhould conclude it to be the moſt com⯑mon manner of reproduction, as it is in reality. Beings of the moſt perfect organization, as ani⯑mals, whoſe bodies make a whole that is inca⯑pable of diviſion, and whoſe powers are all con⯑centrated into one point, cannot be reproduced in any other way; becauſe they contain only particles that are perfectly ſimilar, and cannot be united but by means of different particles furniſhed by another individual. But vegetables, which are leſs perfect in their organization, and which can be divided without deſtruction, are capable of being reproduced in different ways: 1. Becauſe they contain diſſimilar particles; 2. Becauſe the form of theſe bodies is leſs fixed and determined than that of an animal, diffe⯑rent parts may ſupply the functions of each o⯑ther, [284] and vary according to circumſtances: The roots of a tree, when expoſed to the air, puſh out branches and leaves; and thus the organic par⯑ticles of vegetables obtain a local eſtabliſhment, become fixed, and produce individuals in many different ways.

The ſame phaenomenon is exhibited in ani⯑mals whoſe organization is leſs perfect, as in the freſh water polypus, and others, that are capable of reproducing by the diviſion of their parts. Theſe organized bodies, inſtead of ſingle ani⯑mals, may be conſidered as bundles of organized beings united by a common membrane, as trees are compoſed of an infinite number of minute trees^*. The vine-fretters, which propagate indi⯑vidually, likewiſe contain diſſimilar particles; becauſe, after producing their young, they change into barren flies. Snails mutually com⯑municate diſſimilar particles to each other; and, therefore, each individual is fruitful. Thus, in every mode of generation with which we are acquainted, we find, that the neceſſary union of the organic particles cannot be effected but by the admixture of different particles, to ſerve as a common baſis, and to fix or deſtroy their ac⯑tivity.

According to this general idea of ſexes, we may ſuppoſe, that the diſtinction of ſex extends through all Nature; for ſex, in this ſenſe, is no⯑thing but that part of bodies which furniſhes [285] organic particles of a different kind from thoſe of the common parts, and which ſerve as a baſis for their union. But it is, perhaps, uſeleſs to reaſon on a queſtion which can be ſolved at once, by ſaying, that, as God has created ſexes, ani⯑mals muſt neceſſarily be produced by their in⯑tervention. We are not in a condition, as I formerly remarked, to explain why things exiſt; we are unable to explain why Nature almoſt univerſally employs ſexes for the reproduction of animals, or why ſexes exiſt. We ought, therefore, to content ourſelves with reaſoning concerning things as they are. If we attempt to riſe higher, we loſe ourſelves in the regions of fancy, and forget the narrow limits of our knowledge.

Leaving, therefore, all farther ſubtleties, I ſhall adhere to nothing that is not founded on facts and obſervation. I find that the repro⯑duction of bodies is effected in many different modes: But, at the ſame time, I clearly perceive, that animals and vegetables are reproduced by the union of the organic particles detached from all parts of their bodies. I am certain that theſe active organic particles exiſt in the ſeeds of ve⯑getables, and in the ſeminal fluids of animals, both male and female; and have no doubt that every ſpecies of reproduction is accompliſhed by the union and admixture of theſe particles. It is equally unqueſtionable, that, in the gene⯑ration of man, and other animals, the organic [286] particles of the male and female mix at the time of conception; becauſe we often ſee children who reſemble both father and mother: And, what confirms this theory is, that all the particles common to the two ſexes mix together promiſ⯑cuouſly, but that the particles peculiar to the ſexes never mix; for we daily perceive children with eyes reſembling thoſe of the father, while their mouth and front reſemble thoſe of the mother. But we never ſee any ſuch mixture of reſemblances in the ſexual parts; we never find, in the ſame individual, the teſticles of the father and the vagina of the mother.

The formation of the foetus, therefore, is ef⯑fected by the mixture of the organic particles of both ſexes; and this mixture fixes or gives a lo⯑cal eſtabliſhment to the particles, becauſe it is made according to the laws of affinity which take place between the different parts, and which determine the particles to arrange themſelves in the ſame order they obſerved when they exiſted in the individuals who furniſhed them. The particles which proceed from the head, for ex⯑ample, cannot, according to theſe laws, take up their ſtation in the legs, or in any other part but the head of the foetus. All the particles are in motion when they firſt unite; and this mo⯑tion muſt be round the point or centre of union. This baſis or central point, which is neceſſary to the union of the particles, and which, by its reaction and inertia, fixes and deſtroys their ac⯑tivity, [287] is probably the firſt aſſemblage of parti⯑cles that proceed from the ſexual parts, becauſe they are the only particles in the mixture that differ from thoſe common to both ſexes.

I imagine, therefore, that, in the mixture of the two fluids, the organic particles which come from the ſexual parts of the male fix themſelves firſt, and cannot unite with thoſe which pro⯑ceed from the ſeminal parts of the female, be⯑cauſe they are of a different nature, and have leſs affinity to each other than the particles that come from the eye, the arm, or any other part of the female. Round this centre, or point of union, the other organic particles ſucceſſively ar⯑range themſelves in the ſame order they former⯑ly exiſted in the body from which they were de⯑rived; and, according as the particles of the one or of the other individual moſt abound, or near⯑er this central point, they enter in greater or leſs quantities into the compoſition of the new being, which, in this manner, is formed in the middle of a homogeneous fluid; at the ſame time, veſſels begin to ſhoot, which increaſe in proportion to the growth of the foetus, and fur⯑niſh it with proper nouriſhment. Theſe veſſels, which have a peculiar ſpecies of organization, are probably formed by the exceſs of the orga⯑nic particles which have not been admitted into the compoſition of the foetus; for, as theſe particles are both active, and furniſhed with a baſe or point of union from the organic particles pecu⯑liar [288] to the ſexual parts of the other individual, they muſt arrange themſelves in the form of an organized body, but not in the form of another foetus; becauſe their poſition with reſpect to each other has been changed by the different movements of the particles which entered into the compoſition of the firſt foetus. From the union of theſe ſuperabundant particles, therefore, an irregular body muſt ariſe, which will reſemble the foetus in nothing but its growth and expan⯑ſion; becauſe, though this body be compoſed of the ſame organic active particles with the foetus, their poſition and form muſt be different, as they were thrown aſide from the centre or point of union, which ſerved as a baſis for the formation of the foetus.

When the quantity of ſeminal fluid of both individuals is great, or, rather, when theſe liquors abound with organic particles, different centres of attraction are formed in different parts of the mixture; and, in that caſe, by a mechaniſm ſi⯑milar to what has been mentioned above, ſeveral foetuſes are formed, ſome of them males, and others females, according as the particles of the one ſex or of the other are moſt active. But, from the ſame centre of attraction, two foetuſes can never originate; becauſe two centres are re⯑quiſite for this purpoſe. Beſides, if this were to happen, no particles would be left for the for⯑mation of the placenta and membranes; becauſe they would all be employed in conſtituting the [289] ſecond foetus, which would neceſſarily be a fe⯑male, if the other was a male. All that could happen in ſuch a caſe would be, that ſome of the particles common to both individuals, being equally attracted by the firſt centre of union, muſt arrive there at the ſame time, and produce a monſter, or a foetus with ſuperfluous parts; or, if ſome of the common particles ſhould fix at too great a diſtance from the firſt centre, or be conſtrained by the attraction of the ſecond, round which the placenta is formed, a monſter, defec⯑tive in ſome part, would be the conſequence.

That the organic particles peculiar to the ſex⯑ual parts ſerve for a baſis or centre of union to the other particles of which the embryo is form⯑ed, I pretend not to demonſtrate: But, as they are the only particles which differ from the reſt, it is more natural to imagine that they ſhould anſwer this purpoſe than thoſe which are com⯑mon to both individuals.

I formerly detected the errors of thoſe who maintained, that the heart, or the blood, were firſt formed. The whole is formed at the ſame time. We learn from actual obſervation, that the chicken exiſts in the egg before it is ſit upon. The head, the back-bone, and even the appen⯑dages which form the placenta, are all diſtinguiſh⯑able. I have opened a great number of eggs, both before and after incubation, and I am con⯑vinced, from the evidence of my own eyes, that the whole chicken exiſts, in the middle of the [290] cicatrice, the moment the egg iſſues from the bo⯑dy of the hen. The heat communicated to it by incubation, only expands the parts, by putting the fluids in motion. But we have never been able to determine, with certainty, what parts of the foetus are firſt fixed, at the moment of its formation.

I have always maintained, that the organic particles were fixed, and that they united in con⯑ſequence of their motion being ſuſpended. Of the truth of this fact I am fully convinced; for, if the male and female ſemen be ſeparately exa⯑mined, we find in both a great number of mo⯑ving particles; but, when theſe fluids are mix⯑ed, the motion of the particles is totally deſtroy⯑ed, and a certain degree of heat is neceſſary to renew their activity; for the chicken, which ex⯑iſts in the centre of the cicatrice, has no motion before incubation; and, even 24 hours, or two days afterwards, when we begin to perceive it without the microſcope, it has not the ſmalleſt appearance of motion. During the firſt two or three days, the foetus is only a ſmall white mu⯑cilaginous maſs, which gradually acquires con⯑ſiſtence and magnitude. But this progreſs is ex⯑ceedingly ſlow, and has no reſemblance to the rapid movements of the organic particles in the ſeminal fluid. Beſides, it was not without rea⯑ſon that I maintained that the motion of the or⯑ganic particles was entirely deſtroyed; for, if eggs be kept without expoſing them to the de⯑gree [291] of heat that is neceſſary for the expan⯑ſion of the chicken, the embryo, though com⯑pleatly formed, will remain without any motion, and the organic particles of which it is compo⯑ſed will continue fixed, without being able to give life and motion to the embryo which was formed by their union. Thus, after the motion of the organic particles is ſtopped, and after they have united in ſuch a manner as to form a foe⯑tus, ſome external cauſe is ſtill neceſſary to give them life and motion. This cauſe, or agent, is heat, which, by rarifying the fluids, obliges them to circulate: This circulation makes all the or⯑gans act; and nothing farther is neceſſary for the growth and expanſion of the parts than the continuation of this heat.

Before the action of this external heat, there is not the ſmalleſt appearance of blood; and I never could perceive any change of colour in the veſſels till about 24 hours after incubation. In the veſſels of the placenta, which communi⯑cate with the body of the foetus, the blood firſt appears. But, it would ſeem that this blood lo⯑ſes its red colour, as it approaches the body of the animal; for the chicken is entirely white; and, during the firſt, ſecond, and third days af⯑ter incubation, we can with difficulty perceive a few red particles near the animal's body, but which ſeem not to make any part of it, though theſe red particles are deſtined for the formation of the heart. Thus the formation of blood is an effect produced by the motion communicated [292] to the fluids by heat; and even this blood is formed without the body of the animal, the whole ſubſtance of which conſiſts of a white mucilage.

The foetus, as well as the placenta, receive the nouriſhment that is neceſſary for their expanſion by a ſpecies of abſorption; and they aſſimilate the organic particles of the liquor in which they ſwim: For, it is an equal impropriety to ſay that the placenta nouriſhes the animal, as that the a⯑nimal nouriſhes the placenta. If the placenta nouriſhed the foetus, the former would diminiſh in proportion to the growth of the latter, which by no means happens; for both augment toge⯑ther. I have indeed obſerved, that, in eggs, the placenta at firſt grows much more in proportion than the foetus, and, conſequently, it may nou⯑riſh the animal; or, rather, it conveys nouriſh⯑ment to it by means of abſorption.

What we have ſaid concerning the chicken, admits of an eaſy application to the human foe⯑tus, which is formed by the union of the orga⯑nic particles of the two ſexes. The membranes and placenta are produced by the ſuperabundant particles that enter not into the compoſition of the foetus, which is now incloſed in a double membrane, containing alſo a quantity of fluid. This fluid is at firſt, perhaps, nothing but a por⯑tion of the ſeminal liquors of the father and mo⯑ther; and, as the foetus is not thrown out of the uterus, it enjoys, from the moment of its forma⯑tion, [293] as much external heat as is ſufficient for its expanſion. This heat communicates motion to the fluids; it gives play to all the organs; and the blood is formed in the placenta, and in the body of the foetus, ſolely by the motion excited by the heat. We might even maintain, that the formation of the blood in a child is as indepen⯑dent of the mother, as that which ariſes in the egg is independent of the hen that covers it, or of the furnace which heats it.

It is certain that the foetus, the membranes, and the placenta, are all nouriſhed and expanded by abſorption; for, at firſt, the ſack that con⯑tains the whole product of generation does not adhere to the uterus: And we have ſeen, from the experiments of De Graaf upon female rab⯑bits, that he made the globules which contained the foetuſes roll about in the uterus. They could receive no nouriſhment, therefore, but by abſorbing the fluids that conſtantly bedew the uterus, to which they afterwards begin to ad⯑here by means of a mucilage that gradually gives origin to ſmall blood-veſſels, as ſhall afterwards be more fully explained.

But to return to the formation of the foetus, concerning which we have to make ſeveral re⯑marks, both as to its ſituation, and to the differ⯑ent circumſtances that may prevent or alter the mode of its production.

In the human ſpecies, the ſemen of the male enters into the uterus, the cavity of which is [294] conſiderable; and, when it meets with a ſuffi⯑cient quantity of female ſemen, the two inſtantly mix, and the organic particles unite and form the foetus. The whole is, perhaps, performed in a moment, eſpecially if the two fluids be in an active ſtate. The cavity of the uterus is the proper place for the formation of the foetus; becauſe the ſemen of the male has an eaſier ad⯑miſſion into the uterus than into the Fallopian tubes or ovarium; and, as the uterus has only a ſmall aperture, which is always ſhut, excepting when opened by the ardor of love, the materi⯑als of generation remain there in ſafety, unleſs they be diſturbed by ſome rare and accidental circumſtance. But, as the male fluid moiſtens the vagina, before it penetrates the uterus, and, as the organic particles are exceedingly active, it may penetrate as far as the Fallopian tubes and ovarium. In the ſame manner, as the fe⯑male fluid is already perfected in the glandular bodies of the teſticles, from which it diſtills and moiſtens the Fallopian tubes, before it deſcends into the uterus; and, as it may eſcape through the lacunae round the neck of the uterus, it is not impoſſible that the mixture of the two li⯑quors may take place in all theſe different pla⯑ces. Foetuſes, therefore, may frequently be formed in the vagina, and inſtantly fall out of it, having nothing proper for their retention. They may alſo be ſometimes formed in the Fal⯑lopian [295] tubes; but inſtances of this kind muſt be rare.

Anatomiſts mention, that foetuſes have been found, not only in the Fallopian tubes, but like⯑wiſe in the ovaria. M. Theroude^*, a ſurgeon in Paris, ſhowed to the academy a rude maſs which he found in the right ovarium of a girl of 18 years of age. Two open fiſſures, garniſhed with hair like the eye-lids, were perceived in it. Above theſe eye-lids was a kind of front, with a black line in place of eye-brows. Immediate⯑ly above the front, there were ſeveral hairs col⯑lected into two ſeparate pencils, one of them a⯑bout ſeven inches long, and the other three. Below the angle of the eye, two large, hard, white dentes molares appeared, together with their gums: Theſe teeth were about three lines long, and about a line diſtant from each other. Se⯑veral other teeth appeared, ſituated at different diſtances. In the ſame volume^†, M. Mery is reported to have found in the ovarium of a wo⯑man an upper jaw-bone, with ſeveral teeth in it, ſo perfect that they appeared to be at leaſt of ten years growth. In the Medical Jour⯑nal, publiſhed by the Abbé de la Roque^‡, we have the hiſtory of a woman, who died of her ninth child, which had been formed in or near one of the ovaria; for, from the deſcription, it is not clear whether the child was within the o⯑varium, [296] or only adjacent to it. This foetus was about an inch thick, and compleatly formed. In the Philoſophical Tranſactions, examples are re⯑corded of teeth, hair, and bones being found in the ovaria of women. If all theſe facts can be credited, the ſeminal liquor of the male muſt be ſuppoſed ſometimes, though rarely, to mount up to the ovaria. But there are many conſiderations which render this point extremely doubtful: 1. The facts which ſeem to ſupport it are few: 2. The only inſtance of a perfect foetus found in the ovarium, is narrated in a very ſuſpicious manner by M. Littre. Neither is it impoſſible that the ſeminal fluid of the female alone may ſometimes produce organized maſſes, as moles, ciſts full of hair, of bones, or of fleſh. Beſides, if we are to believe anatomiſts, foetuſes may be formed in the teſticles of men as well as in thoſe of women; for, we are told by a ſurgeon, in the 2d volume of the hiſtory of the old academy^*, that he found a foetus, with its membranes, in which the head, the feet, the eyes, the bones and cartilages, were diſtinguiſhable in the ſcrotum of a man. Were all theſe facts equally worthy of credit, we muſt neceſſarily adopt one of the two following hypotheſes; either that the ſeminal fluid of each ſex can produce nothing without being mixed with the other; or, that either of the fluids alone is capable of producing irregu⯑larly organized maſſes. If we maintain the for⯑mer [297] hypotheſis, to explain the facts above rela⯑ted, we ſhall be obliged to admit, that the male fluid ſometimes aſcends to the ovarium, and, by mixing there with the female fluid, forms or⯑ganized bodies; and alſo, that the female fluid, by being copiouſly effuſed in the vagina, may, in the time of coition, penetrate as far as the ſcrotum of the male, in the ſame manner as the venereal virus often reaches that part; and, con⯑ſequently, that an organized body may be form⯑ed in the ſcrotum by a mixture of the male and female fluids. If the other hypotheſis, which is the moſt probable, be adopted, namely, that the ſeminal fluid of each individual may ſeparately produce organized maſſes, then all theſe oſſeous, fleſhy, and hairy bodies, which ſometimes appear in the ovaria of females, and in the ſcrotum of males, may derive their origin from the ſeminal fluid of the individual in which they are found. But it is needleſs to ſpeculate farther concerning facts which ſeem to be more uncertain than in⯑explicable; for I am inclined to think, that the ſeminal fluid of each individual may ſingly pro⯑duce ſomething; and that young girls, for ex⯑ample, may produce moles, without any inter⯑courſe with the male, in the ſame manner as hens lay eggs without the intervention of the cock. I might ſupport this opinion with obſer⯑vations equally credible as thoſe we have juſt now quoted. M. de la Saone, a phyſician and anatomiſt, publiſhed a treatiſe on this ſubject, in [298] which he aſſures us, that nuns, though ſtrictly cloiſtered, ſometimes produce moles: And why ſhould this be impoſſible, ſince hens produce eggs without any communication with the cock, and ſince we find, in the cicatrices of theſe eggs, in⯑ſtead of a chicken, a mole with its appendages? The analogy here is ſufficiently ſtrong to make us at leaſt ſuſpend a raſh determination. What⯑ever be in this, it is certain, that a mixture of the two fluids is neceſſary for the formation of a foetus, and that this mixture cannot be pro⯑perly effected but in the uterus, or in the Fallo⯑pian tubes, where anatomiſts have ſometimes diſ⯑covered foetuſes: And it is natural to imagine, that thoſe which have been found in the cavity of the abdomen, have eſcaped by the extremity of the tube, or by ſome accidental rupture of the uterus; and that they never fall into the abdo⯑men from the ovarium, becauſe I think it next to impoſſible that the ſeminal fluid ſhould aſcend ſo far. Leeuwenhoek has computed the motion of his pretended ſpermatic animals to be four or five inches in 40 minutes; ſo that, if the whole fluid moved at this rate, in an hour or two the animalcules might proceed from the va⯑gina into the uterus, from the uterus into the Fallopian tubes, and from the Fallopian tubes into the ovaria. But, how is it poſſible to con⯑ceive, that the organic particles, whoſe motion ceaſes whenever they are deprived of the fluid part of the ſemen, ſhould arrive at the ovarium, [299] unleſs they were accompanied with the liquor in which they ſwim? The moving particles cannot give a progreſſive motion to the fluid which contains them. Thus, whatever activity may be aſcribed to theſe organic particles, we cannot conceive how they ſhould arrive at the ovarium, and there form a foetus, unleſs, by ſome unknown power, the fluid is abſorbed by the ovarium, a ſuppoſition which is not only gra⯑tuitous, but contrary to probability.

The difficulty attending this ſuppoſition con⯑firms the opinion, that the male fluid enters the uterus, either by its orifice, or by penetrating its ſubſtance. The female fluid may likewiſe find its way into the uterus, either by the aperture at the extremity of the Fallopian tubes, or by pe⯑netrating the ſubſtance of the tubes and uterus. M. Weitbrech, an able anatomiſt of the acade⯑my of Peterſburg, has clearly proved that the ſe⯑minal fluid can penetrate through the ſubſtance of the uterus: 'Res omni attentione digniſſima,' ſays he, 'oblata mihi eſt in utero feminae alicu⯑jus a me diſſectae; erat uterus ea magnitudi⯑ne qua eſſe ſolet in virginibus, tubaeque ambae apertae quidem ad ingreſſum uteri, ita ut ex hoc in illas cum ſpecillo facile poſſem tranſire ac flatum injicere, ſed in tubarum extremo nul⯑la debatur apertura, nullus aditus; fimbriarum enim ne veſtigium quidem aderat, ſed loco il⯑larum bulbus aliquis pyriformis materia ſubal⯑bida [300] fluida turgens, in cujus medio fibra plana nervea, cicatriculae acmula, apparebat, quae ſub ligamentuli ſpecie uſque ad ovarii involu⯑cra protendebatur.'

'Dices, eadem a Regnero de Graaf jam olim notata. Equidem non negaverim illuſ⯑trem hunc proſectorem, in libro ſuo de organis mulieribus, non modo ſimilem tubam delineaſſe, Tab. xix. ſig. 3. ſed et monuiſſe "tubas, quam⯑vis, ſccundum ordinariam naturae diſpoſitionem, in extremitate ſua notabilem ſemper coarcta⯑tionem habeant, practer naturam tamen ali⯑quando claudi;" verum enimvero cum non meminerit auctor an id in utraque tuba ita de⯑prehenderit, an in virgine, an ſtatus iſte prae⯑ternaturalis ſterilitatem inducat, an vero con⯑ceptio nihilominus fieri poſſit, an a principio vitae talis ſtructura ſuam originem ducat, ſive an tractu temporis ita degenerare tubae poſſint, facile perſpicimus multa nobis relicta eſſe pro⯑blemata, quae, utcumque ſoluta, multum nego⯑tii faceſcant in exemplo noſtro. Erat enim haec foemina maritata, viginti quatuor annos nata, quae filium pepererat quem vidi ipſe, octo jam annos natum. Dic igitur tubas ab incun [...] clauſas ſterilitatem inducere: Quare haec nostra foemina peperit? Dic concepiſſe [...]: Quomodo ovulum ingredi tubam potuit? Dic [...]aluiſſe tubas post partum: Quomodo id nost [...] quomodo adeo evaneſcere in [301] utroque latere fimbriae poſſunt, tanquam nun⯑quam adfuiſſent? Si quidem ex ovario ad tu⯑bas alia daretur via, praeter illarum orificium, unico greſſu omnes ſuperarentur difficultates; ſed fictiones intellectum quidem adjuvant, rei veritatem non demonſtrant; praeſtat igitur ig⯑norationem fateri, quam ſpeculationibus in⯑dulgere^*.' Theſe difficulties, which occurred to this acute obſerver, are inſurmountable, accor⯑ding to the ovicular ſyſtem. But the fact he re⯑cords is alone ſufficient to demonſtrate, that the female fluid may penetrate the ſubſtance of the uterus; and it is not to be doubted that the male fluid is capable of entering it in the ſame man⯑ner. The change which the male fluid produ⯑ces in the uterus, and the ſpecies of vegetation or expanſion which it occaſions in that viſcus, is ſufficient to demonſtrate the truth of the fact. Beſides, the fluid which iſſues through the la⯑cunae of De Graaf being of the ſame nature with that of the glandular bodies, it is evident that this liquor proceeds from the ovaria; and yet there are no veſſels through which it could paſs. We muſt therefore conclude, that it pene⯑trates through the ſpongy ſubſtance of the parts, and that it not only enters the uterus, but may even iſſue out of it, when the parts are irritated.

But, though this penetration ſhould be re⯑garded as impoſſible, it cannot be denied, that the female fluid, which diſtills from the glandu⯑lar [302] bodies of the ovaria, may fall into the uterus, by entering the apertures at the extremities of the Fallopian tubes, as the male fluid enters by the orifice of the uterus itſelf; and, conſequent⯑ly, that the foetus may be formed in the cavity of the uterus, by the mixture of the two fluids, in the manner already explained.

CHAP. XI.
Of the Expanſion, Growth, and Delivery of the Foetus, &c.

IN the expanſion of the foetus, two different ſpecies of growth are diſtinguiſhable: The firſt, which immediately ſucceeds the formation of the foetus, is not uniform in all the parts of the animal. The nearer the foetus approaches to maturity, the growth of the parts is more proportional; and it is not till after the birth, that all the parts grow nearly in an equable manner. We muſt not imagine, that the foe⯑tus, at the time of its formation, has the exact figure of an adult. The ſmall embryo, it is true, contains all the parts eſſential to a man; but they differ in their ſucceſſive expanſion.

In an organized body, like that of an animal, ſome parts may be ſuppoſed to be more eſſential than others; and though none of them are uſe⯑leſs or ſuperfluous, yet there are ſome to which others ſeem to owe their growth and diſpoſition. Some parts may be conſidered as fundamental, without which the animal could not exiſt, and others as only ſuperficial and acceſſory. The [304] latter ſeem to derive their origin from the for⯑mer, and to be intended more for conferring ſymmetry and external ornament on the animal, than for enabling it to exiſt, or to perform the functions eſſential to life. Theſe two different ſpecies of parts are ſucceſſively expanded, and are almoſt equally apparent at the time of birth. But there are other parts, as the teeth, which arrive not at full maturity till ſeveral years af⯑ter; and others, as the glandular bodies in the female teſticles, the beard of males, &c. which appear not till the age of puberty.

To diſcover the fundamental and eſſential parts of an animal body, attention muſt be had to the number, ſituation, and nature of the whole. Thoſe which are ſimple, thoſe whoſe poſition is invariable, and thoſe without which the animal cannot exiſt, muſt neceſſarily be the moſt eſſential. Thoſe, on the contrary, which are double, or more numerous, thoſe which vary in ſize and poſition, and thoſe which may be taken away without injuring or killing the creature, may be regarded as leſs eſſential, or more acceſſory to the animal machine. Ariſtotle mentions, that the only parts eſſential to all ani⯑mals are thoſe with which they take and digeſt their nouriſhment, and throw out the ſuperflu⯑ous part of it from the body. The whole in⯑teſtinal canal is indeed extremely ſimple, and no other part can ſupply its place. The head and back-bone are likewiſe ſimple parts, the po⯑ſition [305] of which is invariable. The back-bone is the foundation of the animal frame; and the action and movements of moſt members of the body depend upon the ſpinal marrow which that bone contains. It is this part, alſo, together with the head, which appears firſt in the embryo. Now, theſe ſimple parts, which are firſt formed, are all eſſential to the exiſtence and the form of the animal.

The double parts in an animal body are more numerous than the ſingle parts; and they ſeem to be produced on each ſide of the ſingle parts by a ſpecies of vegetation; for the double parts are ſimilar in form, and different only in poſition. The left hand is perfectly ſimilar to the right; but, if the left hand was placed in the ſituation of the right, we could not perform the ſame ac⯑tions with it. The ſame thing may be obſerved of all the double parts: They are ſimilar in form, but differ in poſition, which is relative to the body of the particular animal; and, if a line were drawn, dividing the body into two equal parts, the poſition of all the ſimilar parts would tend to this line as to a centre.

The ſpinal marrow, and the vertebrae in which it is incloſed, appear to be the real axis of all the double parts of the animal body, from which they ſeem to derive their origin, and to be only proportional branches iſſuing from this trunk or common baſe; for, in the young chick, we ſee the ribs ſhooting from each ſide of the [306] vertebrae, as the ſmall branches ſhoot out from the principal branch of a tree. In all embryos, the middle of the head and the vertebrae firſt appear; we then ſee, on each ſide of the veſicle which compoſes the middle of the head, two o⯑ther veſicles which ſeem to proceed from the firſt: Theſe two veſicles contain the eyes, and the other double parts of the head. In the ſame manner, we perceive an equal number of ſmall tubercles iſſuing from each ſide of the vertebrae, which gradually extend, and form the ribs, and other double parts of the trunk. Laſtly, the arms and legs appear like ſmall tubercles on each ſide of the trunk. This firſt growth is very different from what afterwards takes place: It is the production of parts which appear for the firſt time; the growth which ſucceeds is on⯑ly an expanſion of parts already formed.

The order and ſymmetry of the double parts in all animals, their regular poſition, the equa⯑lity of their extenſion and growth, and the per⯑fect ſimilarity of their ſtructure, ſeem to indi⯑cate, that they derive their origin from the ſin⯑gle parts; that a certain force reſides in the ſin⯑gle parts that acts equally on each ſide; or, which amounts to the ſame, they are the baſes or fulcra againſt which the action of thoſe powers that produce the expanſion of the double parts is exerted; and that the action of theſe forces, both on the right and left ſides, are preciſely equal, and counterbalance each other.

[307] Hence we may conclude, that, if there is any defect, or redundance, in the matter deſtined for the formation of the double parts, as the forces are equal on each ſide, this defect or exceſs muſt take place both in the right and left ſides. If, for example, from a defect of matter, a man has but two fingers in the right hand, in place of five, the ſame defect will appear in the left; if, from an exceſs of matter, there be ſix fingers in the right hand, he will have the ſame number in the left; or, if the matter be vitiated, and produces a change in the parts of one ſide, the ſame change will appear in the other. Of this fact we have daily proofs: The parts of monſters are always deranged in a certain order and proportion. Hence Nature, even in her er⯑rors, uniformly commits the leaſt of poſſible miſtakes.

This harmony in the poſition of the double parts of animals, is likewiſe apparent in vege⯑tables. The branches puſh out ſmaller ramifi⯑cations on each ſide; the ſmall nerves in the leaves are equally diſpoſed with regard to the principal nerve; and, if the ſymmetry appears to be leſs exact in vegetables than in animals, this proceeds only from its being more various, and becauſe its limits are more extenſive, and leſs preciſe. But the ſame order is eaſily recog⯑niſable; and the ſingle and eſſential parts are perfectly diſtinct from thoſe which are double; [308] and it is evident, that the latter derive their ori⯑gin from the former.

It is impoſſible to determine the form of theſe double parts before their expanſion, or in what manner they are complicated, or what figure reſults from their poſition in relation to the ſingle parts. The body of an animal, at the inſtant of its formation, unqueſtionably con⯑tains all the parts of which it ought to be com⯑poſed: But the relative diſpoſition of theſe parts is then very different from what after⯑wards appears. If we examine the expanſion of a young leaf of a tree, we will find that it is plaited on each ſide of the principal nerve; and that its figure, at this time, has no reſemblance to that which it afterwards aſſumes. When we amuſe ourſelves with plaiting paper, in order to give it the form of a crown, of a boat, &c. the different plaits of the paper ſeem to have no re⯑ſemblance to the figure which reſults from their expanſion: We only perceive that the plaits are uniformly made in a certain order and pro⯑portion, and that, whatever is done on one ſide, is alſo done on the other. But, to determine the figures which may reſult from the expan⯑ſion of any given number of folds, is a problem beyond the powers of geometry. The ſcience of mathematics reaches not what immediately depends upon poſition. Leibnitz's art of Ana⯑lyſis ſitus does not yet exiſt; though the art of knowing the relations that reſult from the poſi⯑tion [309] of things would be, perhaps, more uſeful than that which has only magnitude for its ob⯑ject; for we have more occaſion to be acquaint⯑ed with form than with matter.

In the expanſion of natural productions, the folded or plaited parts not only aſſume different poſitions, but they acquire, at the ſame time, ex⯑tenſion and ſolidity. Since, therefore, we are unable to aſcertain the exact reſult of a ſimple expanſion of a folded figure, in which, as in folded paper, nothing takes place but a change of poſition among the parts, without any aug⯑mentation or diminution of the quantity of matter, how is it poſſible for us to judge con⯑cerning the expanſion of the compound body of an animal, in which not only the relative poſi⯑tion, but likewiſe the quantity of matter in theſe parts, ſuffer conſiderable changes? We can only reaſon, therefore, on this ſubject, by drawing concluſions from the examination of objects at different periods of their expanſion.

We, indeed, perceive the chick in the egg before incubation: It ſwims in a tranſparent fluid, contained in a ſmall purſe formed by a very fine membrane in the centre of the cica⯑trice. But the chick is then only a particle of inanimated matter, in which no organization or determined figure can be diſtinguiſhed. We can perceive, however, that one of its extremities is the head, and the other the back-bone. The embryo, in this ſtate, ſeems to be the firſt pro⯑duct [310] of fecundation, reſulting from the mixture of the male and female ſemen. To aſcertain this fact, ſeveral things require attention: When the hen has, for ſome days, been along with the cock, and afterwards ſeparated from him, the eggs produced 20 days or a month after this ſe⯑paration, are equally fertile as thoſe laid during her cohabitation with the male. The eggs pro⯑duced at the end of this period require only the uſual time of 21 days in hatching; and their embryos are equally advanced both in form and conſiſtence. From this circumſtance we might be led to imagine, that the form in which the embryo appears before incubation, is not the immediate effect of the mixture of the two ſe⯑minal fluids, but that it exiſted in different forms during the abode of the egg in the body of the mother; for the embryo, in the form in which we ſee it before incubation, requires only the aid of heat in order to bring it to maturity. Now, if this form of the embryo had exiſted 21 days or a month before, when the egg was firſt impregnated, why was it not hatched by the internal heat of the mother? Why do we not find the chick compleatly formed in thoſe eggs which have been impregnated 21 days be⯑fore they are laid?

But this difficulty, though ſeemingly great, is not inſurmountable. When the hen cohabits with the cock, the cicatrice of each egg, which contains the ſemen of the female, receives a ſmall [311] quantity of the male fluid. The eggs in the o⯑varium of oviparous females are analogous to the glandular bodies in the teſticles of the vivipa⯑rous: The cicatrice of the egg correſponds to the cavity of thoſe glandular bodies which con⯑tain the female ſemen; and that of the male pe⯑netrates and mixes with it. The formation of the embryo inſtantly reſults from this mixture or union of the two fluids. The firſt egg laid by the hen after her communication with the cock, is fecundated and capable of being hatch⯑ed. Thoſe which ſhe is afterwards to lay were all impregnated at the ſame inſtant: But, as they want ſome eſſential parts, the production of which has no dependence on the male fluid; as they have neither the white, the membranes, nor the ſhell, the ſmall embryos contained in the cicatrices of theſe imperfect eggs, are inca⯑pable of being hatched, though aided by the in⯑ternal heat of the mother. The embryo, there⯑fore, remains in the cicatrice, in the ſame ſtate in which it was firſt formed, till the egg acquires all the parts neceſſary to the growth and nou⯑riſhment of the foetus; and it is not till after the egg has arrived at full perfection, that the ex⯑panſion of the embryo commences. This ex⯑panſion is effected by the external heat of incu⯑bation: But it is unqueſtionable, that, if the egg could be retained in the body 21 days after it was completely formed, the chick would be hatched, unleſs the internal heat of the mother [312] was too great; for the degrees of heat neceſſa⯑ry for hatching eggs are very limited; and the leaſt exceſs or defect is equally fatal to this ope⯑ration. The laſt eggs, therefore, laid by the hen, containing the embryos in the ſame ſtate as the firſt, prove nothing more than the neceſſi⯑ty of their acquiring full perfection before they can be hatched.

It is apparent, therefore, that the ſtate of the embryo when the egg is laid by the hen, is its firſt ſtate, and that which immediately ſucceeds impregnation; that it undergoes no interme⯑diate changes of form; and conſequently, by tracing, as Malpighius has done, its gradual ex⯑panſion, hour after hour, we diſcover every thing that it is poſſible for us to know, unleſs we could perceive the mixture of the two fluids, and the manner in which the particles arrange themſelves during the firſt formation of the em⯑bryo.

If we reflect on this inſtantaneous fecundation of a number of eggs, which are to be laid at ſucceſ⯑ſive intervals, we ſhall diſcover a new argument againſt the exiſtence of eggs in viviparous ani⯑mals: For, if women contained eggs, like hens, why are not many of them fecundated at the ſame time? Why does not one impregnation give birth to a ſucceſſive race of children? And, when women conceive two or three children, why do they always come into the world at the ſame time? If theſe foetuſes were produced [313] from eggs, would they not ſucceed each other according to the different ſtates of perfection of the ſeveral eggs at the time of impregnation? And, would not ſuperfoetations be as frequent as they are rare, and as natural as they are acci⯑dental?

It is impoſſible to trace the gradual expanſion of the human foetus, as we can that of the chick in the egg. The opportunities for obſervation are few; and all we know of this ſubject is de⯑rived from the writings of anatomiſts, ſurgeons, and accoucheurs. It is from collecting all their particular obſervations, and comparing their re⯑marks with their deſcriptions, that the following abridged hiſtory of the human foetus has been compiled.

Immediately after the mixture of the two ſe⯑minal fluids, it is probable that the whole mate⯑rials of generation exiſt in the uterus under the form of a ſmall globe; for we learn from ana⯑tomiſts, that, three or four days after conception, there is a ſmall globular maſs in the uterus, the greateſt diameter of which is about 6 lines, and the leaſt 4. This globe is formed by a de⯑licate membrane, which contains a limpid li⯑quor very like the white of an egg. We may already perceive, in this liquor, ſome ſmall fi⯑bres, which are the firſt rudiments of the foetus. Upon the ſurface of the globe there is a net⯑work of delicate fibres, which extends from one [314] of the extremities to the middle: Theſe are the firſt veſtiges of the placenta.

Seven days after conception, the lineaments of the foetus are diſtinguiſhable by the naked eye. They are, however, very imperfect, and have the appearance of a gelly almoſt tranſparent, though it has acquired ſome degree of ſolidity. The head and trunk may be eaſily diſtinguiſh⯑ed; becauſe this maſs is of an oblong figure, and the trunk is longeſt and moſt delicate. Some ſmall fibres, reſembling a plume of feathers, iſ⯑ſue from the middle of the foetus, and termi⯑nate in the membrane in which it is incloſed. Theſe fibres are the rudiments of the umbilical cord.

Fifteen days after conception, the head and the moſt prominent features of the face are ap⯑parent. The noſe reſembles a ſmall elevated thread perpendicular to a line which marks the di⯑viſion of the lips: Two ſmall black points repre⯑ſent the eyes; and we ſee two holes in place of ears. The body of the foetus has alſo acquired ſome growth. On each ſide of the ſuperior and in⯑ferior parts of the trunk, thoſe ſmall protube⯑rances appear, which are the rudiments of the arms and legs. The length of the whole body is about five French lines.

Eight days after, in all three weeks, the body of the foetus is only augmented about a line: But the arms and legs are apparent. The growth of the arms is quicker than that of the [315] legs; and the fingers ſeparate ſooner than the toes. The internal organization now begins to be viſible: The bones appear like fine threads. The ribs are diſpoſed on each ſide of the back-bone like minute threads: The arms, the legs, the fingers and toes, are alſo repreſented by ſi⯑milar threads.

At one month, the foetus is more than an inch in length: It naturally aſſumes a curved poſture in the middle of the liquor that ſur⯑rounds it; and the membranes in which the whole is included, are both augmented and thickened. The whole maſs is of an oval figure, the greateſt diameter of which is about an inch and a half, and the leaſt about an inch and a quarter. The human figure is no longer equi⯑vocal: All the parts of the face are already diſtinguiſhable; the body is delineated; the haunches and the belly are prominent; the hands and legs are formed, and their fingers and toes are divided; the ſkin is thin and tranſpa⯑rent; the viſcera reſemble a knot or plexus of ſibres; the veſſels are like fine threads, and the membranes are extremely delicate: The bones are ſtill ſoft, and it is only in a few places that they have begun to aſſume ſome degree of ſoli⯑dity. The veſſels which form the umbilical cord lie parallel to each other in a ſtraight line. The placenta now occupies only a third of the whole maſs, inſtead of a half, which it did during the firſt days; the ſuperficial growth of the placenta, [316] therefore, has not been ſo great as that of the foetus and maſs; but it has received a great augmentation in ſolidity; it has become pro⯑portionally thicker than the membranes of the foetus, both of which are now diſtinguiſhable.

According to Hippocrates, the male foetus expands ſooner than the female.

At the end of ſix weeks, the foetus is about two inches long, and the human form begins to be more perfect, only the head is very large in proportion to the other parts of the body. A⯑bout this time the motion of the heart becomes viſible: In 50 days, the heart has been percei⯑ved to beat for a conſiderable time after the foetus was extracted from the uterus.

In two months, the foetus is more than two inches in length; and the oſſiſication is per⯑ceptible in the middle of the two arm-bones, in the thigh and leg, and in the point of the under jaw, which is then greatly advanced before the upper. Theſe, however, are only oſſeous points. But, by means of a quicker growth, the clavi⯑cles are entirely oſſified: The umbilical cord is formed, and the veſſels which compoſe it begin to twiſt like the threads of a rope: But this cord is very ſhort in compariſon of the length it afterwards acquires.

In three months, the foetus is nearly three inches long, and weighs about three ounces. Hippocrates aſſerts, that at this time the motions of the male foetus begin to be felt by the mo⯑ther; [317] but that thoſe of the female are not per⯑ceptible till four months. Some women, how⯑ever, affirm that they have felt the motions of the foetus at the beginning of the ſecond month. It is difficult to acquire any certain knowledge on this ſubject: The ſenſations excited by the firſt movements of the foetus depend more, per⯑haps, on the ſenſibility of the mother, than the ſtrength of the child.

Four months and a half after conception, the length of the foetus is from ſix to ſeven inches. All the parts are greatly augmented, and eaſily diſtinguiſhable from each other: Even the nails appear on the fingers and toes. The teſticles of the male are ſhut up in the belly above the kid⯑neys. The ſtomach is filled with a thick fluid, ſimilar to that which is contained in the amnios. In the ſmall guts we find a milky fluid, and a black liquid matter in the great guts. There is a ſmall quantity of bile in the veſica felis, and a little urine in the bladder. As the foetus floats freely in the fluid that ſurrounds it, there is always ſome ſpace between its body and the membranes in which it is contained. Theſe membranes, at firſt, grow more rapidly than the foetus; but, after a certain time, the reverſe takes place. Before the end of the third month, the head is bent forward; the chin reſts on the breaſt; the knees are elevated, and the legs folded back upon the thighs. One of the hands, and often both, touch the face. Afterwards, [318] when the foetus acquires more ſtrength, it per⯑petually changes its poſition, as we learn from the following obſervations made by perſons ſkil⯑led in the art of midwiſery: 1. The umbilical cord is often twiſted round the body and mem⯑bers of the child, in a manner that neceſſarily ſuppoſes different motions and poſitions. 2. Mo⯑thers feel the motion of the child ſometimes on one ſide of the uterus, and ſometimes on the o⯑ther; and it often ſtrikes againſt many different places, which could not happen unleſs it aſſumed different poſitions. 3. As the foetus ſwims in a fluid which ſurrounds it on all ſides, it may ea⯑ſily turn, extend, and twiſt itſelf by its own powers: It muſt likewiſe take different ſituations, according to the various attitudes of the mo⯑ther's body; when the mother, for example, lies down, the poſition of the foetus muſt differ from what it is when ſhe ſtands.

Moſt anatomiſts maintain, that the foetus is obliged to bend its body, becauſe it is too much confined by the membranes. But this opinion ſeems not to be well founded; for, during the firſt five or ſix months, at leaſt, there is room more than ſufficient to admit a full extenſion of the foetus; and yet, during all this period, the foetus is bended. We ſee likewiſe, that the chick is bended in the liquor of the amnios, while, at the ſame time, this membrane, and the fluid it contains, afford room ſufficient to hold a body five or ſix times larger than the foetus. We [319] may, therefore, conclude, that the bended po⯑ſture of the foetus is natural, and not the effect of reſtraint. I am inclined to think, with Harvey, that the foetus takes this poſture, be⯑cauſe it is moſt favourable to reſt and ſleep; for all animals bend their bodies when they want to repoſe themſelves, or to ſleep: And, as the foetus ſleeps almoſt continually, it naturally aſ⯑ſumes this advantageous poſition. 'Certe,' ſays this celebrated anatomiſt, 'animalia omnia, dum quieſcunt et dormiunt, membra ſua ut pluri⯑mum adducunt et complicant, figuramque o⯑valem ac conglobatam quaerunt: Ita pariter embryones qui aetatem ſuam maxime ſomno tranſigunt, membra ſua poſitione ea qua plaſ⯑mantur (tanquam naturaliſſima ac maxime in⯑dolenti quietique aptiſſima) componunt^*.'

The uterus, as formerly remarked, grows very quickly after pregnancy; and it continues to increaſe proportionally with the foetus. But the growth of the foetus at laſt exceeds that of the uterus; and it is natural to think, that the foetus, when near maturity, is too much con⯑fined, and agitates the uterus by repeated mo⯑tions. The mother feels thoſe ſucceſſive efforts, which are called periodic pains, after the la⯑bour of child-bearing commences. The more force the foetus exerts in order to dilate the u⯑terus, it finds an increaſed reſiſtance from the natural elaſticity of the parts. Hence every ef⯑fort [320] tends to open the os tincae, or orifice of the uterus, which has already been gradually en⯑larged during the laſt months of pregnancy. The head of the foetus puſhes along againſt the margins of this orifice, and dilates it by a con⯑ſtant preſſure, till the moment of delivery, when it opens ſo wide as to allow a free paſſage to the child.

What renders it probable that the labour-pains are occaſioned by the dilatation of the os tincae is, that this dilatation is the only certain mode of diſtinguiſhing the real from the falſe pains. Women often feel very briſk pains, which are not thoſe that immediately precede delivery. To diſtinguiſh thoſe falſe from the true pains, Deventer adviſes the accoucheur to feel the orifice of the uterus, and maintains, that, if the pains be true, the dilatation will augment upon the occaſion of every pain; and that, on the contrary, if the pains be falſe, the orifice will rather contract than dilate, or, at leaſt, that it will not continue to dilate. Hence we may conclude, that theſe pains proceed not from a forced dilatation of the orifice of the uterus. The only thing that is embarraſſing, is the alter⯑nation of pain and of eaſe experienced by the mother. This circumſtance does not perfectly correſpond with the cauſe we have aſſigned; for the gradual and continued dilatation of an orifice ſhould produce a conſtant pain, without any intervals of eaſe. Perhaps the alternations [321] may be aſcribed to the ſeparation of the placen⯑ta: It adheres to the uterus by the inſertions of a number of ſmall papillae. May we not, therefore, ſuppoſe that theſe papillae ſeparate not from their cavities all at once; and that the ſucceſſive ſeparation of theſe papillae gives riſe, at different intervals, to the freſh acceſſions of pains? Here the effect perfectly correſponds with the cauſe; and this conjecture may be ſup⯑ported by another remark: Immediately before delivery, there iſſues out a viſcous whitiſh li⯑quor, ſimilar to that which flows from the pa⯑pillae of the placenta, when torn from the u⯑terus. It is therefore extremely probable, that this liquor, which iſſues from the uterus, is pro⯑duced by the ſeparation of ſome of the papillae of the placenta.

It ſometimes happens, that the foetus eſcapes from the uterus without burſting the membranes, and, conſequently, without diſcharging the li⯑quor they contain. This ſpecies of birth ſeems to be the moſt natural, and is ſimilar to that of moſt animals. The human foetus, however, commonly pierces the membranes, by the re⯑ſiſtance it meets with at the orifice of the uterus: And ſometimes a part of the amnios, and even of the chorion, is brought away adhering to the head of the child like a cap. As ſoon as the membranes are pierced or torn, the liquor, which is called the waters, runs out, and, by lubrica⯑ting the vagina and orifice of the uterus, facili⯑tates the paſſage of the child. After the diſcharge [322] of the waters, there is ſufficient room left in the uterus for the midwife to return the child, when its poſition is unfavourable to the birth. After the child comes into the world, the delivery is not completed. The placenta and membranes ſtill remain in the uterus; and the child is at⯑tached to them by means of the umbilical cord: They are eaſily brought away by the hand of the midwife; and ſometimes the weight of the child is ſufficient for that purpoſe. Theſe or⯑gans, which were neceſſary to the exiſtence of the foetus, become uſeleſs, and even noxious to the child, after its birth. They are, therefore, inſtantly diſengaged from the child's body, by caſting a knot on the umbilical cord, about an inch from the navel, and by cutting the cord an inch above the ligature. In ſix or ſeven days, the remains of the cord dry up, and fall off cloſe to the navel.

By examining the foetus before birth, we are enabled to form ſome ideas concerning the me⯑chaniſm of its natural functions. There are or⯑gans neceſſary to it while in the womb of the mother, but which become uſeleſs immediately after birth. The better to comprehend theſe functions, we muſt explain more fully the na⯑ture of theſe acceſſory parts, the umbilical cord, the membranes, with the liquor they contain, and the placenta. The umbilical cord, which is attached to the body of the foetus at the navel, is compoſed of two arteries and a vein: By [323] theſe the courſe of the circulation is lengthened; but the vein is larger than the arteries. At the extremity of the cord, each of theſe veſſels di⯑vide into an infinite number of ramifications, and extend themſelves between two membranes. They ſet off from the common trunk in ſuch a manner, that the whole ramifications aſſume a round form, and are diſtinguiſhed by the name of placenta, becauſe they reſemble a cake. The central part of the placenta is thicker than its edges: Its mean thickneſs is about an inch, and its diameter is eight or nine inches, and ſome⯑times more. The external ſurface of it, which is applied to the uterus, is convex, and the in⯑ternal ſurface is concave. The blood of the foetus circulates in the cord and in the placenta. The arteries of the cord proceed from two large arteries in the foetus, and carry the blood through all the arterial ramifications of the pla⯑centa; and the blood is collected and returned to the foetus by the venous branches of the pla⯑centa and the umbilical vein.

The concave ſurface of the placenta is cover⯑ed with the chorion: Its convex ſurface is alſo covered with a ſoft membrane, which ſeems to be a continuation of the chorion, and is eaſily torn; and the foetus is incloſed in the double covering of the chorion and amnios. The figure of the whole is globular; becauſe the intervals between the foetus and membranes are filled with a tranſparent fluid. This liquor is imme⯑diately [324] confined by the amnios, which is the in⯑ternal membrane: It is thin and tranſparent, and folds itſelf round the umbilical cord, at its inſertion into the placenta, and continues to co⯑ver it the whole way to the navel of the foetus. The chorion is the external membrane; it is thick, ſpongy, and interſperſed with blood-veſ⯑ſels. It conſiſts of ſeveral coats, the outermoſt of which covers the convex ſurface of the pla⯑centa. It ſends off duplicatures to cover the papillae, which are inſerted into the cavities at the fundus of the uterus, called lacunae. Theſe inſertions connect the foetus to the uterus.

Some anatomiſts have maintained, that the human foetus, like thoſe of certain quadrupeds, was furniſhed with an allantois, a membrane deſtined for the reception of the urine; and they have pretended to have diſcovered it between the chorion and amnios, or in the middle of the placenta, at the root of the umbilical cord, un⯑der the form of a pretty large bladder; and that it received the urine by means of a long tube which made a part of the cord, and which open⯑ed at one end into the bladder, and, at the others, into the allantois, anſwering the ſame purpoſes as the urachus in other animals. They acknow⯑ledge, however, that the urachus of the human foetus is not near ſo large as in the quadrupeds; but they aſſert that it is divided into a number of ſmall tubes, and that the urine paſſes into their cavities.

[325] To theſe facts are oppoſed the experience and obſervation of moſt anatomiſts. They ſeldom find any veſtiges of an allantois either between the chorion and amnios, or in the placenta; nor do they perceive any urachus in the umbi⯑lical cord. A kind of ligament, indeed, runs from the external ſurface of the bottom of the bladder to the navel; but, when entering the cord, it becomes ſo delicate as to be almoſt re⯑duced to nothing. Neither is this ligament commonly hollow; and we can perceive no cor⯑reſponding aperture in the bottom of the blad⯑der.

The foetus has no communication with the open air; and the experiments made upon the lungs demonſtrate that they have never reſpired; for they ſink in water, while thoſe of infants, who have breathed, uniformly ſwim: The foe⯑tus, therefore, has no reſpiration in the womb of the mother; conſequently, it can make no ſound with its voice, and all the ſtories of chil⯑dren groaning and crying before birth muſt be regarded as fabulous. After the waters run off, however, the air may find admiſſion into the cavity of the uterus, and the child may begin to reſpire before its birth. In this caſe, the child may cry, in the ſame manner as the chick⯑en cries before the ſhell of the egg is broken, which it is enabled to do by means of the air lodged in a cavity between the external mem⯑brane and the ſhell: This air exiſts in all eggs, [326] and is produced by the fermentation of the matters they contain^*.

The lungs of the foetus, having no motion, receive no more blood than is ſufficient for their nouriſhment and growth: Another paſſage, therefore, is open for its circulation. The blood in the right auricle of the heart, inſtead of paſ⯑ſing into the pulmonary artery, and, after cir⯑culating through the lungs, returning into the left auricle by the pulmonary vein, paſſes direct⯑ly from the right to the left auricle, through an aperture called the foramen ovale, which is in the partition of the heart that ſeparates the two auricles: The blood then enters the aorta, by the ramifications of which it is diſtributed to every part of the body; it is then taken up by the numerous branches of the veins, which gra⯑dually unite into one trunk, called the vena cava, that terminates in the right auricle of the heart. The blood contained in this auricle does not all paſs through the foramen ovale; part of it e⯑ſcapes into the pulmonary artery, but it enters not into the body of the lungs; for there is a communication between the pulmonary artery and the aorta, by an arterial canal which leads immediately from the one to the other. It is by theſe means that the blood circulates in the foetus, without entering the lungs, which it does in children, in adults, and in all animals who reſpire.

[327] It has been imagined by ſome, that the blood of the mother paſſes into the body of the foetus, by means of the placenta and umbilical cord: They ſuppoſed, that the blood-veſſels of the u⯑terus opened into the lacunae, and thoſe of the placenta into the papillae, and that they inoſcu⯑lated with one another. But this opinion is contradicted by experiment. When the arteries of the umbilical cord are injected, the liquor re⯑turns by the veins, without any of it eſcaping externally. Beſides, the papillae can be drawn out of the lacunae in which they are lodged, without any extravaſation of blood either from the uterus or placenta; from both there oozes out a milky matter, which, we have already re⯑marked, ſerves for the nouriſhment of the foe⯑tus. It is probable that this liquor enters the veins of the placenta in the ſame manner as the chyle enters the ſubclavian vein; and the pla⯑centa, perhaps, performs the office of the lungs in maturating the blood. One thing is certain, that the blood appears much ſooner in the pla⯑centa than in the foetus; and I have often ob⯑ſerved, in eggs which had been ſit upon for a day or two, that the blood appeared firſt in the membranes, and that their blood-veſſels are nu⯑merous and large, while the whole body of the foetus, excepting the point where theſe blood-veſſels terminate, is only a white tranſparent matter, in which there is not the leaſt veſtige of blood.

[328] It has been imagined that the liquor of the amnios is a nouriſhment received by the mouth of the foetus. Some have even pretended to have found this liquor in the ſtomach, and to have ſeen ſeveral foetuſes who wanted the um⯑bilical cord entirely, and others who had only a ſmall portion of it, which had no connection with the placenta. But, in this caſe, may not the liquor have paſſed into the body of the foe⯑tus by the portion of the cord that remained, or even by the navel itſelf? Beſides, other facts may be oppoſed to theſe: Foetuſes have been found, whoſe lips were not ſeparated; and o⯑thers whoſe oeſophagus had no aperture. To reconcile theſe facts, ſome anatomiſts have main⯑tained, that the aliment paſſed into the foetus partly by the umbilical cord, and partly by the mouth. But none of theſe opinions ſeem to have any foundation. The queſtion is not, how the foetus alone, but how the whole apparatus of generation, receive their growth and nouriſh⯑ment? for the placenta, the liquor, and the membranes, increaſe in bulk as well as the foe⯑tus; and, conſequently, thoſe inſtruments and canals employed for receiving and tranſporting nouriſhment to the foetus, are themſelves en⯑dowed with a ſpecies of life. The expanſion of the placenta and membranes is equally diffi⯑cult to conceive as that of the foetus; and, it may be ſaid, with equal propriety, that the foe⯑tus nouriſhes the placenta, as that the placenta [329] nouriſhes the foetus. At the commencement of growth, the whole maſs floats in the uterus, without any adheſion; and, of courſe, the nou⯑riſhment can only be conveyed by an abſorption of the lacteous fluid contained in the uterus. The placenta appears firſt to attract this fluid, which it converts into blood, and tranſports by the veins into the foetus. The liquor amnii ſeems to be nothing but this milky fluid in a purified ſtate, the quantity of which is augmented, by a ſimilar abſorption, in proportion to the growth of the membranes; and the foetus probably ab⯑ſorbs this liquor, which ſeems to be neceſſary for its growth and nouriſhment: For, it is worthy of remark, that the foetus, during the firſt two or three months, contains very little blood: It is as white as ivory, and appears like a conge⯑ries of lymph ſomewhat conſolidated; and, as the ſkin is tranſparent, and all the parts extreme⯑ly ſoft, the body of the foetus may be eaſily pe⯑netrated by the fluid in which it ſwims, and thus receive the matter neceſſary for its growth and expanſion. It may indeed be ſuppoſed that the foetus afterwards receives nouriſhment by the mouth; becauſe we find a liquor, ſimilar to that of the amnios, in the ſtomach, urine in the blad⯑der, and meconium, or excrement, in the inte⯑ſtines; and, as neither urine nor meconium ap⯑pear in the amnios, it is natural to conclude, that no excrements are voided by the foetus, e⯑ſpecially as ſome are born without having the [330] anus perforated, and yet large quantities of me⯑conium are found in their inteſtines.

Though the foetus has no immediate connec⯑tion with the uterus, but is only attached to it by the ſmall external papillae of the placenta; though it has no communication with the blood of the mother, but, in ſome meaſure, is equally independent of her as the egg is independent of the hen which covers it; yet it has been main⯑tained, that, whatever affects the mother, pro⯑duces a ſimilar effect upon the foetus, and that the impreſſions received by the former are com⯑municated to the ſenſorium of the latter. To this imaginary influence have been attributed all thoſe reſemblances, monſtroſities, and peculiar marks which appear on the ſkin of particular chil⯑dren. Many of theſe marks I have examined, and they uniformly appeared to be occaſioned only by a derangement in the texture of the ſkin. Every mark muſt neceſſarily have a faint reſemblance to ſomething or other: But ſuch reſemblances, I am perſuaded, depend more on the imagination of thoſe who ſee them, than up⯑on that of the mother. On this ſubject, the marvellous has been puſhed to an extreme de⯑gree. The foetus has not only been ſaid to bear the real repreſentations of the appetites of the mother, but that, by a ſingular ſympathy, the marks which repreſent ſtrawberries, cherries, &c. aſſumed a deeper colour during the ſeaſon of theſe fruits. A little attention, however, will [331] convince us, that theſe changes of colour are more frequent, and that they happen whenever the motion of the blood is accelerated, whether it be occaſioned by the heat of ſummer, or by any other cauſe. The marks are always either yellow, or red, or black; becauſe the blood gives theſe colours to the ſkin when it enters in too great quantities into the veſſels. If theſe marks were occaſioned by the appetites of the mother, why are not their forms and colours as various as the objects of her deſires? What a multitude of ſtrange figures would be exhibited, if all the whimſical longings of a mother were written upon the ſkin of the child?

As our ſenſations have no reſemblance to the objects which excite them, it is impoſſible that deſire, fear, horror, or any other paſſion or emo⯑tion, can produce real repreſentations of the ob⯑jects by which they are occaſioned. An infant being, in this reſpect, equally independent of the mother, as the egg is independent of the hen that ſits upon it, I ſhould be equally induced to believe, that the imagination of a hen, which ſaw by accident a cock's neck twiſted, ſhould produce wry-necked chickens from the eggs ſhe was hatching, as that a woman, who ſaw a man broke upon the wheel; ſhould produce, by the mere force of imagination, a child with all its limbs broken.

But, ſuppoſing this fact to be well atteſted, I ſtill maintain that the imagination of the mother [332] could not be the cauſe of it: For, what is the effect of horror? An internal movement, or, if you pleaſe, a convulſion of the mother's body, which might alternately compreſs and ſtretch the uterus. What would be the reſult of this commotion? Nothing ſimilar to its cauſe; for, if the commotion was very violent, the foetus might be killed, wounded, or have ſome of its parts deranged: But how is it poſſible to ima⯑gine that this commotion ſhould produce in the foetus any thing ſimilar to the thoughts of the mother, unleſs we ſuppoſe, with Harvey, that the uterus poſſeſſes the faculty of conceiving ideas, and of realiſing them upon the foetus?

But, if the imagination of the mother has no effect upon the foetus, it may ſtill be demanded, Why did this child come into the world with its members broken? Though a direct ſolution of a fact, which is both extraordinary and uncer⯑tain, is not to be expected; yet, I think, this queſtion admits of a ſatisfactory anſwer. Phae⯑nomena of the moſt uncommon kind, and which are but rarely exhibited, as neceſſarily happen, as thoſe that are uſual and frequent. Among the infinite combinations of which matter is ca⯑pable of forming, arrangements of the moſt pe⯑culiar and extraordinary ſpecies muſt ſometimes take place. Hence, out of the numberleſs chil⯑dren which daily come into the world, one may ſometimes appear with two heads, with four legs, or with all its members broken. It is, [333] therefore, within the circle of nature, that a child, without the aid of the mother's imagina⯑tion, may be born with its arms and legs bro⯑ken. This phaenomenon may have been exhi⯑bited oftener than once; the mother of this child may, during her pregnancy, have ſeen a man broken on the wheel; and the defects of conformation in the child may have been attri⯑buted to the impulſe made, by this dreadful ſpectacle, upon the imagination of the woman. But, independent of this general ſolution, the fact may be explained in a more direct manner. The foetus, as formerly remarked, has nothing in common with the mother. Its functions, its organs, its blood, its movements, are all peculiar, and belong to itſelf alone. The only matter it derives from the mother, is the liquor or nutri⯑tive lymph which diſtills from the uterus. If this lymph ſuffers any change, if it be infected with the venereal virus, the infant is affected with the ſame diſeaſe; and it is reaſonable to think that all the diſeaſes which proceed from viciated humours may be communicated from the mother to the child. We know that the ſmall pox is communicated in this manner; and we have too many examples of children, imme⯑diately after birth, becoming innocent victims of the debauchery of their parents. The veneral virus attacks the moſt ſolid parts of the bones; and it appears to act with more force upon the middle of the bones, which is the part where the [334] oſſification firſt commences, and is, of courſe, the moſt hard and ſolid part. I conceive, there⯑fore, that the infant in queſtion has been affect⯑ed with the venereal diſorder while in the womb of its mother, and that this was the reaſon why it came into the world with its bones broken through the middle.

The ſame effect might be produced by the rickets: In the royal cabinet, there is a ſkeleton of a rickety child, the bones of whoſe legs and arms are joined in the middle by a callus: From inſpecting this ſkeleton, it appears that its bones had been broken before birth, and afterwards re⯑united by a callus.

But we have dwelt too long upon a fact which credulity alone has rendered marvellous. Pre⯑judice, eſpecially that ſpecies of it which is founded in wonder, will always triumph over reaſon. It is needleſs to attempt to perſuade women that the marks on their children have no connection with their ungratified longings. I have ſometimes aſked them, before the birth of a child, of what particular longings they had been diſappointed, and, of courſe, what marks the child would bear? But I had only the ſatis⯑faction of perplexing, without convincing them.

The time of geſtation is generally about nine months; but it is ſometimes longer and ſome⯑times ſhorter. Many children are born in the ſeventh and eighth, and ſome not till after the ninth month: But, in general, the births before [335] the ninth month are more frequent than thoſe that exceed that term.

It is generally believed, that children born in the eighth month cannot live, or, at leaſt, that more of them die than of thoſe who come into the world in the ſeventh month. This opinion appears to be paradoxical; and, if we conſult experience, I believe it will be found to be er⯑roneous. A child born in the eighth month is more perfectly formed, and conſequently more vigorous and lively, than one who is born in the ſeventh. This opinion, however, is very com⯑monly received, and is founded on the authori⯑ty of Ariſtotle: 'Caeteris animantibus ferendi uteri unum eſt tempus, homini vero plura ſunt; quippe et ſeptimo menſe et decimo naſcitur, atque etiam inter ſeptimum et decimum poſitis; qui enim menſe octavo naſcuntur, etſi minus, tamen vivere poſſunt^*.' The beginning of the ſeventh month is the earlieſt term of delivery. If the foetus be rejected ſooner, it dies, and is denominated an abortion. Thus the time of geſtation is more various in the human ſpecies than in other animals; for it extends from the 7th to the 10th, and, perhaps, to the eleventh month.

We are aſſured by women who have had ma⯑ny children, that females remain longer in the womb than males. If this be true, it is not ſur⯑priſing that female children ſhould ſometimes be [336] born in the 10th month. When infants come into the world before the 9th month, they are neither ſo large nor ſo well formed as thoſe who appear not till a later period. Thoſe, on the contrary, who remain in the womb till the 10th month, are larger and better made; their hair is longer; the growth of the teeth, though ſtill concealed within the gums, is more advanced; and the tone of their voice is deeper and more diſtinct.

With regard to the occaſional cauſes of deli⯑very, there is much uncertainty. It is imagined by ſome writers, that, when the foetus has ac⯑quired a certain ſize, the capacity of the uterus becomes too ſmall for its retention, and that the reſtraint felt by the child obliges it to exert every effort to break its priſon. Others alledge, which amounts nearly to the ſame thing, that the foe⯑tus becomes too heavy to be ſupported by the uterus, which, therefore, opens to be diſcharged of its load. Neither of theſe reaſons appear to be ſatisfactory: The uterus has always ſufficient capacity and ſtrength to contain and ſupport the weight of a child of nine months; for it is often loaded with two, during the ſame period; and it is certain, that the weight and ſize of two chil⯑dren of eight months, for example, exceed thoſe of a ſingle infant of the ſame age. Beſides, it is not unfrequent that a child of nine months is leſs than another at eight months, though it ſtill remains in the womb.

[337] Galen pretends that the foetus continues in the uterus till it is able to take nouriſhment by the mouth, and that the want of proper food makes it reſtleſs, and anxious to eſcape. It has been ſaid by others, that the foetus is originally nouriſhed by the mouth, but that, in proceſs of time, the liquor amnii is ſo contaminated with the urine and tranſpiration of the foetus, that it becomes perfectly diſguſtful, and obliges the child to uſe every method to effect its eſcape from the womb.

Theſe reaſons ſeem not to be more ſatisfactory than the former; for from them it would fol⯑low, that the ſmalleſt and weakeſt foetuſes would neceſſarily remain longer in the womb than thoſe of larger and more robuſt bodies; which is by no means the caſe. Beſides, it is not for nouriſhment that the child, immediately after birth, ſeems to be anxious; for it can diſpenſe with the want of it for a conſiderable time after: It appears, on the contrary, to be extremely de⯑ſirous of eaſing itſelf of the ſuperfluous load of nouriſhment (the meconium) received in the womb. This circumſtance induced Drelincourt, and ſome other anatomiſts, to think, that the acrimony and uneaſineſs, ariſing from an ac⯑cumulation of excrement in the bowels, is the reaſon why they become reſtleſs, and uſe every effort to eſcape from the womb. I am not, I acknowledge, more ſatisfied with this explication [338] than the others. If the child is preſſed with faeces, why does it not evacuate them in the liquor amnii? But this never happens. It ap⯑pears, on thè contrary, that the neceſſity of eva⯑cuating the meconium is not felt till after birth, when the motion of the diaphragm, occaſioned by reſpiration, compreſſes the inteſtines, and gives riſe to this evacuation; eſpecially ſince no meconium was found in the amnios of a foetus of ten months, who had not reſpired, and ſince an infant of ſix or ſeven months diſcharges the meconium ſoon after reſpiration.

Other anatomiſts, and particularly Fabricius ab Aquapendente, imagined that the foetus left the uterus, from a deſire of being refreſhed by reſpiration. But this cauſe ſeems to be as chi⯑merical as any that has been mentioned. It is impoſſible that a foetus can have any idea of reſpiration; and far leſs can it have any concep⯑tion whether reſpiration would be agreeable or diſagreeable.

After conſidering all theſe hypotheſes, I ſu⯑ſpect that the delivery of the foetus depends on a cauſe of a very different nature. The men⯑ſtrual flux returns at ſtated intervals. Though its appearance be interrupted by impregnation, its cauſe is not deſtroyed; and, though no blood is exhibited at the accuſtomed period; yet a re⯑volution in the ſyſtem, ſimilar to what happens before impregnation, muſt take place. It is for [339] this reaſon, that, in ſome women, the menſes are not entirely ſuppreſſed during the firſt two or three months after conception. I imagine, therefore, that this periodic revolution happens as regularly after a woman has conceived as be⯑fore; but that the blood is prevented from flowing, by the excretories of the uterus being ſwelled and ſhut up, unleſs when it arrives in ſuch large quantities, and acts with ſuch force, as to overcome the reſiſtance which is oppoſed to it. In this caſe, a great quantity of blood ruſhes out, and an abortion is the conſequence. But it frequently happens, that a ſmall quantity of blood appears, without producing this effect; becauſe the blood has only been able to open a few of the canals or excretories of the uterus, while the reſt remain entirely obſtructed.

Though no blood appears, which is generally the caſe, the firſt revolution fails not to be ac⯑companied with the ſame painful ſymptoms. During the firſt ſuppreſſion of the menſes, therefore, the uterus is affected with a conſide⯑rable agitation, which, when a little augmented, entirely deſtroys the product of generation. Hence, we may reaſonably conclude, that few of thoſe conceptions, which happen a ſhort time before the accuſtomed return of the menſes, are ſucceſsful; becauſe the action of the menſtrual blood eaſily deſtroys the feeble roots of a germ ſo tender and ſo delicate. Thoſe conceptions, on the contrary, which take place immediately [340] after this periodic diſcharge, ſucceed much bet⯑ter; becauſe the foetus is allowed more time to grow, and to fortify itſelf againſt the action of the blood, when the next revolution happens in the ſyſtem.

After the foetus has been enabled to reſiſt the action of the firſt revolution, the increaſe of its growth, and of its attachment to the uterus, ren⯑der it ſtill more capable of reſiſting any of the ſubſequent revolutions: Abortions, indeed, ſometimes happen during every revolution; but they are more rare in the middle period of ge⯑ſtation, than either at the beginning or near the end of it. Why they are more frequent at the beginning, has already been explained: It only remains to ſhow why they are likewiſe more frequent towards the end.

The foetus generally comes into the world du⯑ring the tenth revolution of the menſes. When it is born at the ninth or eighth, it lives, and is not, therefore, regarded as an abortion. Some have pretended to have ſeen inſtances of chil⯑dren born at the ſeventh, and even at the ſixth revolution, who, notwithſtanding this unfavour⯑able circumſtance, continued to live. There is no difference between abortion and birth, but what relates to the living powers of the child. In general, the number of abortions in the firſt, ſecond, and third months, for the reaſons already aſſigned, is very great; and the number of pre⯑ [...] births, in the ſeventh and eighth months, [341] is alſo very great, in proportion to the abortions in the fourth, fifth, and ſixth months; becauſe, during this middle term of geſtation, the pro⯑duct of generation having acquired ſtrength and ſolidity ſufficient to reſiſt the action of the firſt four periodic revolutions, a more violent effort than any of the former is neceſſary to deſtroy it. For the ſame reaſon, an abortion is more difficult during the fifth and ſixth months. But the foetus, which till now was weak, and could only exert its own force in a feeble manner, begins to move with more vigour; and, when the eighth revolution takes place, the efforts of the foetus uniting with thoſe of the uterus, and facilitating its excluſion, the foetus may come into the world in the ſeventh month, and be in a capacity of living, whenever it happens to be unuſually ſtrong at this period. But, if the foe⯑tus be excluded ſolely from a weakneſs of the uterus, which renders it unable to reſiſt the ac⯑tion of the blood during the eighth revolution, the birth of it is conſidered as an abortion, and the child dies. But ſuch caſes are uncommon; for, it the foetus has reſiſted the firſt ſeven re⯑volutions, nothing but particular accidents can prevent it from reſiſting the eighth, unleſs it has acquired more vigour than is common at this pe⯑riod. A foetus which has acquired the ſame degree of ſtrength, but at a later period, will be excluded at the ninth revolution; and thoſe which require nine months in obtaining this degree of [342] ſtrength, will be born at the tenth revolution, which is the moſt uſual term. But, when the foetus acquires not this degree of ſtrength and perfection in nine months, it will remain in the uterus till the eleventh, or even the twelfth re⯑volution, that is, till the tenth or eleventh month: Of ſuch late births many examples are record⯑ed.

Other reaſons, to confirm the opinion, that the menſtrual flux is the occaſional cauſe of births at different periods, may be produced. The females of all animals which have no men⯑ſes, bring forth very nearly at the ſame terms: The difference in the times of geſtation is ex⯑tremely ſmall. We may, therefore, conclude, that this variation, which is very great in wo⯑men, proceeds from the action of the menſtrual blood, which is exerted at every periodic revo⯑lution.

We have already remarked, that the placenta adheres to the uterus only by the papillae; that there is no blood either in theſe papillae, or in the lacunae in which they are inſerted; and that, when they are ſeparated, an operation which requires no great effort, a milky liquor only iſ⯑ſues from them. Why, therefore, is the birth of a child uniformly followed by a conſiderable haemorrhage, firſt of pure blood, and afterwards of blood mixed with a watery fluid? This blood proceeds not from the ſeparation of the placen⯑ta; for the papillae are drawn out of the lacunae [343] without any effuſion of blood. Delivery, there⯑fore, which is nothing more than this ſepara⯑tion, ought to produce no haemorrhage. Is it not more natural, on the contrary, to think, that the action of the blood is the cauſe of the birth; and that it is this menſtrual blood alone which forces the veſſels of the empty uterus, and be⯑gins to flow immediately after delivery, in the ſame manner as it did before conception?

We know, that, for ſome time after conception, the ſack which contains the product of gene⯑ration, adheres not to the uterus. We have ſeen, from the experiments of De Graaf, that, by blowing upon the ſmall globule, it is made to change its poſition. The adheſion to the uterus is never very ſtrong: In the early periods of geſtation, the placenta is ſlightly applied to the uterus; and thoſe parts are only contiguous, or joined by a mucilaginous matter which has hard⯑ly any adheſion. How, then, ſhould it happen, that, in abortions of the firſt or ſecond month, this globule never eſcapes without being attend⯑ed with a great effuſion of blood? This effuſion cannot be occaſioned by the paſſage of the glo⯑bule, which has no adheſion to the uterus. It is by the action of the blood, on the contrary, that the globule is extruded. Should we not, therefore, conclude, that this is the menſtrual blood, which, by forcing the canals through which it was accuſtomed to flow before impreg⯑nation, [344] deſtroys the product of conception, and reſumes its ordinary courſe?

The pains of child-bearing are principally occaſioned by this action of the blood; for, it is well known, that they are equally violent in abortions of two or three months, as in ordinary births; and that many women feel, without ha⯑ving conceived, very acute pains, whenever the menſtrual flux is about to appear. Theſe pains are of the ſame kind with thoſe which accompany abortions or births. Ought we not, therefore, to aſcribe them to the ſame cauſe?

It appears, then, that the periodic revolution of the menſtrual blood has great influence in child-bearing, and that it is the cauſe why the terms of delivery in women are more various than in other animals which are not ſubject to this diſcharge, and which always bring forth at the ſame times. It is alſo apparent, that the revolution occaſioned by the action of the men⯑ſtrual blood is not the only cauſe of birth: The action of the foetus itſelf contributes greatly to this end; for there are inſtances of children ha⯑ving made their eſcape from the uterus after the death of the mother, which could only happen from an exertion peculiar to the foetus.

The terms of geſtation in cows, ſheep, and other animals, are always the ſame, and no hae⯑morrhage attends their delivery. May we not, therefore, conclude, that the blood diſcharged by women after delivery is the menſtrual [345] blood, and that the action of this blood upon the uterus, during every periodic revolution, is the reaſon why the human foetus is excluded at ſo many different terms? It is natural to i⯑magine, that, if the females of viviparous ani⯑mals had menſes like women, their deliveries would be followed by an effuſion of blood, and be equally various in their terms. The foetuſes of animals are brought forth covered with their membranes; and it is ſeldom that the mem⯑branes are broken, or the waters flow before their delivery. But the birth of a child, with its membranes entire, is a rare phaenomenon. This circumſtance ſeems to evince, that human foetuſes make greater efforts to eſcape from their priſon than thoſe of other animals, or that the uterus of a woman affords not ſo free a paſ⯑ſage to the child; for it is by the ſtruggles of the foetus againſt the reſiſtance it meets with at the orifice of the uterus, that the membranes are torn.