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and developements, with which I have no business in' this place, and which I leave to its own fate. Let us begin, by examining the ground on which the doctrine of animalcules retts: and they are the following. The two authors I have mentioned, affirm, that the spermatic animalcula are found, few or none at all in early youth, in decrepitude, in impotent people, in the excessively incontinent, in high fevers, or in disease. They maintain, that these animalcula are found always in people that are healthy, vigorous and potent, and in the matrix of females that have had connection with the male; whence they think they are entitled to infer, that fea 'cundity resides in the spermatic animal, and that it is this very animal, which is converted into the fætus. M. Andri thinks himself the more justified in this conclusion, that those of man have a head larger than those of other ania mals; as this corresponds to the figure of the human fætus, the head of which is very large, in proportion to the rest, when this fætus is still very small.
But not to multiply circumstances, I shall grant to these authors, that few or no spermatic animalcules are found in those cases, where they pretend they are so rare, or that they are wanting altogether : but I must be permitted to doubt the univerfality of the opposite fact, to wit, that they are always, and without exception, found in all animals capable of generation. A multitude of experiments are neces. sary, to establish such a fact, and the system of Leewenhoek would not perhaps gain much, if they were oftener repeated. It is said, that great and enlightened philosophers, who have wanted to verify these experiments, have not always found living animalcula in the semen of animals perfectly fit for generation; and without going further, Leewenhoek himself, has known many persons in health, of a proper age, and who even had families, and yet, who had no anie malcula. Such experiments would make one doubt the validity of the system in question ; but they do not puzzle its partizans. They have always two answers to give : The experiments have not been properly made ; or, the subject was impotent..
Let us leave them this resource. Supposing it proved that fertile femen alone is full of thele spermatic animalcu. la, what better reason have they to conclude that the animalcula are the cause of fertility, than I have to conclude thap fertility produces the animalcula? May not the semen pro
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per per for generation alone poffefs the quality neceffary for making them multiply abundantly, while in sterile semen, which has not the same quality, they multiply so little that they are hardly discernible in it? A fpecies of fmall ferpent is often generated in vinegar, but never in the wine of which that vinegar is made ; must we therefore conclude that it is the existence of these ferpents in the liquor which makes it vinegar and not wine ; or ought we not rather to conclude that they are found in vinegar only because it is fit for their living and multiplying in ? Stagnant water nou. rishes a vast number of small animals which are not found in fresh water ; shall we conclude that these animals have made the water ftagnant, or that the stagnant water has multiplied the animals ? But to give an instance more nearly connected with the prefint quettion : we know, that certain vermin, which it is indelicate to name, multiply exceeding. ly, in the bodies of such as are of a luxurious temperament, and but little in people of a more temperate constitution, and that it perishes in people afflicted with disease. What are we to infer from this? Will it be said, that it is the vermin which produces the luxurious temperament; or, that it is the temperament that has multiplied the verinin? I imagine, no body will hesitate, in adopting the last opinion; why then thould we think differently with regard to the animalcula we are talking of?
Further, if animalcula were only found in the semen, and in the veffels which prepare it, this fingularity might prepoffefs one in favour of the doctrine of Leewenhoek; but they are found of different sizes in all parts of the body.
Leewenhoek himself discovered exceedingly minute ones, and • of different kinds, under the skin, in the mass of blood, in
the fæces, and even in the foulness of the teeth. These animalcuies, surely were not destined for the multiplication of the individuals of the species on whom they lived; why must those of the spermatic fluid be fo?
But, it is laid, the speridatic animalcula are of a very different nature from those which live upon us.. The first do not injure the health, and are never found, but in vigorous subjcets. The others, on the contrary, are noxious, they bring on diseases; and it is often in disease that they mult ply most. • Suppote the fe facis should be granted, I do not fee that the lyttem of Lewenhoek would gain much. But how is it
known, that the spermatic animalcula are not noxious, and that their exceffive numbers do not generate disorder in the humours, by which they themselves may perish? And though they should not injure the health, need we seek any other reason for this, than their extreme minuteness? Ani. mals, a million of times less than a grain of sand, and which live in a fluid, do not seem calculated to produce much disturbance in the bodies they inhabit ; especially, if we con. sider, that the substance they feed on makes no part of those, bodies, but is separated from them, to answer other purposes, so that these animals do not live at the expence of their hoft. The case is not the same with chofe animals which we know are hurtful; they feed on our substance, they consume the chyle, they attack the nobler parts, and are large enough to produce numberless disorders. Besides, these animals are of many different species. If some are generated in disease, there are others, perhaps, which can only multiply on the sound and healthy. Those worms, . which are sometimes voided by sick persons, are no more a proof that they are generated by disease, than they are a proof, that diseases kill them.
But how is it known, that all animals which live at our ex. pence are hurtful to us? Are there any proofs, that those exceedingly minute animalcula, which are found in the mass of blood, and perhaps in the whole body, have ever done us the least harm? These are the animals, and not those an hundred millions of times larger, which ought to be fet in opposition to the spermatic animalcula, in order to draw a conclusion in favour of the latter.
What has been said, in my opinion, sufficiently shews, that even though fertile semen thould alone be always full of animalcula, it would by no means follow, that they are the cause of the fertility of that substance. As to the resemblance, said to be found between the human spermatic worm and the fætus, the head in both being large in proportion to the rest of the body, I do not see that any great advantage can be made of it. It is not a very conclusive argument to say, the animalcule has a large head, the fætus has a large head, ergo, the animalcule makes the fætus. From the way in which this animalcule is figured, neither its body nor its head, have any resemblance, in external forin, to the head and body of the fætus. It rather resembles a tadpole, consisting of nothing but a head and a
tail. Mr Andri himself makes this comparison. Now, as what is taken for the head of the tadpole, is in reality its whole body, inclosed in an orbicular fpace; may not the fame be the case with the spermatic worm, and then, what becomes of its resemblance to the fætus?
We conclude, from all this, that the system of Leewenboek is not built on any folid foundation : and that there. fore, even though it should not be attended with difficule ties, we ought never to consider it, but as a mere conjecture, which we may admit or reject, as we find convenit nt, and of which, a little more or less probability, makes all the merit.
But this fyftem is very far from being free of difficulties: many might be urged against it, of which the fo'lowing are a few.
I remark, in the first place, that according to the observa. tion of Swammerdam, there must be many forts of animals, even in those, of which it is pretended spermatic animal. cules are found, that by no means owe their origin to thefe worms. M. Leewenhoek maintains, that the femen of insects is full of animalcules, as well as that of other animals. He has been able to discover them in that of beetles, dra. gon-flies, grasshoppers, gnats, and even in fleas; but Swammerdam, who is not used to advance facts on flight grounds, lays it down as certain, that the fætus of infects, from the very formation of the egg, and consequently, long before copulation, fills the whole capacity of the egg. If this be true, it must follow, of necessity, that this fætus does not derive its origin from one of the animacules in the male femen, which 'could not enter into the egg, till long after its formation. Here, then, is a fætus formed, without the aflittance of spermatic worms, and that, even in an animal which poffestes them. Is any thing more neede ed to overturn the System I am examining?.
I obferve, in the second place, that Leewenhoek, in a letter, without date, written to Sir Christopher Wren, and inserted in the collection of his letters, printed in 1696, says positively, that he had found two forts of fpermatic worms in the fame subject, whence he concludes, that the one produces the male, and the other the female.
But, have we not better reason to conclude, that they produce neither? Indeed, if these animalcula differed from one another, only in sex, is it probable that the difference
would be fo great, in animals of such extreme minuteness, as to make them appear animals of two different fpecies? And, if they really are animals of two different species at firit, how can they afterwards become animals of the same fpecies, differing only in fex? ,
My third observation respects the crigin of those small animals. They are not found, according to Leewenhoek, and those who adopt his f; stem, in early youth ; at the age of puberty, their number is prodigious; they almost all perith in disease; they appear again, on the return of health, and the vast quantity,lost at the union of the fexes, is always supplied while the generative faculty remains. From all these facts, we cannot but conclude, that there animalcules are generated in the body which harbours them ; and, if they are there generated, I demand how this takes place. Are they formed there, by an immediate production, or by way of propagation ? If they are formed by an immediate production, we must allow, that there resides in the feminal matier, or in the vessels which form it, a faculty, capable of producing daily, hundreds of millions of living beings, without the assistance of any animalcule; and if so, why may it not be allowed, that the fætus can be produced in the same manner, without the same aslistance, by a similar faculty? But, if it is maintained, that the animalcules in question are multiplied in the spermatic fluid, by the way of propagation, they must not only be fit for generation, long before attaining the age of maturity, and in a state, in which they could hardly be said to have begun to be animals; but, according to the principles of Leewenhoek, we must also allow, that in their femen, there are other ani.. mals, infinitely smaller, to which they owe their origin, as these other animals muít, in their turn,owe theirs to animalcules, smaller ftill, in the same proportion, which might be carried ad infinitum, unless we should find at last some, whose femen had the faculty of fecundating the female, without the aslistance of small animated pre-existing beings. And, if * we must come to that at last, what do we gain by the system of Leewenhoek? And, why not allow the same faculty to the lemen of larger animals?
In the fourth place, if it is maintained, that the fætus is formed of one of these small spermatic animalcules, we must fuppose it to grow, with such surprising rapidity, as, if it is not altogether impoffible, is at leaft incredible, and has