and developements, with which I have no bufinefs in this place, and which I leave to its own fate. Let us begin, by examining the ground on which the doctrine of animalcules. refts: and they are the following. The two authors I have mentioned, affirm, that the fpermatic animalcula are found, few or none at all in early youth, in decrepitude, in impotent people, in the exceffively 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 fe cundity refides in the fpermatic animal, and that it is this very animal, which is converted into the foetus. M. Andri thinks himself the more juftified in this conclufion, that thofe of man have a head larger than those of other ani mals; as this correfponds to the figure of the human fœtus, the head of which is very large, in proportion to the reft, when this fœtus is ftill very fmall.

But not to multiply circumstances, I fhall grant to these authors, that few or no fpermatic animalcules are found in thofe cafes, where they pretend they are fo rare, or that they are wanting altogether: but I must be permitted to doubt the univerfality of the oppofite fact, to wit, that they are always, and without exception, found in all animals capable of generation. A multitude of experiments are neceffary, to eftablish fuch a fact, and the fyftem of Leewenhoek would not perhaps gain much, if they were oftener repeated. It is faid, that great and enlightened philofophers, who have wanted to verify thefe experiments, have not always found living animalcula in the femen of animals perfectly fit for generation; and without going further, Leewenhoek himfelf, has known many perfons in health, of a proper age, and who even had families, and yet, who had no ani malcula. Such experiments would make one doubt the validity of the fyftem in queftion; but they do not puzzle its partizans. They have always two anfwers to give: The xperiments have not been properly made; or, the fubject was impotent.

Let us leave them this refource. Suppofing it proved that fertile femen alone is full of thefe fpermatic animalcu la, what better reason have they to conclude that the animal cula are the cause of fertility, than I have to conclude that fertility produces the animalcula? May not the femen pro

Y y 2

per

per for generation alone poffefs the quality neceffary for making them multiply abundantly, while in fterile femen, which has not the fame quality, they multiply fo little that they are hardly difcernible in it? A fpecies of fmall ferpent is often generated in vinegar, but never in the wine of which that vinegar is made; muft 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 nourifles a vast number of fmall animals which are not found in fresh water; fhall we conclude that these animals have made the water stagnant, or that the stagnant water has multiplied the animals? But to give an instance more nearly connected with the prefent queftion: we know, that certain vermin, which it is indelicate to name, multiply exceedingly, in the bodies of fuch as are of a luxurious temperament, and but little in people of a more temperate conftitution, and that it perithes in people afflicted with difeafe. What are we to infer from this? Will it be faid, that it is the vermin which produces the luxurious temperament; or, that it is the temperament that has multiplied the vermin? I imagine, no body will hefitate, 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 femen, 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 fizes in all parts of the body. Leewenhoek himfelf difcovered exceedingly minute ones, and of different kinds, under the fkin, in the mafs of blood, in the feces, and even in the foulnefs of the teeth. These animalcules, furely were not deftined for the multiplication of the individuals of the fpecies on whom they lived; why muft thofe of the fpermatic fluid be fo?

But, it is faid, the fpermatic 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 vigo rous fubjects. The others, on the contrary, are noxious, they bring on difeafes; and it is often in difeafe that they mult ply most.

Suppofe the fe facts fhould be granted, I do not fee that the fyftem of Leewenhoek would gain much. But how is it

known,

known, that the fpermatic animalcula are not noxious, and that their exceffive numbers do not generate disorder in the humours, by which they themselves may perifh? And though they should not injure the health, need we feek any other reason for this, than their extreme minutenefs? Animals, a million of times lefs than a grain of fand, and which live in a fluid, do not seem calculated to produce much difturbance in the bodies they inhabit; efpecially, if we confider, that the fubftance they feed on makes no part of those bodies, but is feparated from them, to answer other purpofes, fo that these animals do not live at the expence of their hoft. The cafe is not the fame with thofe animals which we know are hurtful; they feed on our fubftance, they confume the chyle, they attack the nobler parts, and are large enough to produce numberlefs diforders. Befides, these animals are of many different fpecies. If some are generated in difeafe, there are others, perhaps, which can only multiply on the found and healthy. Thofe worms, which are fometimes voided by fick persons, are no more a proof that they are generated by disease, than they are a proof, that difeafes kill them.

But how is it known, that all animals which live at our expence are hurtful to us? Are there any proofs, that those exceedingly minute animalcula, which are found in the mafs of blood, and perhaps in the whole body, have ever done us the leaft harm? These are the animals, and not those an hundred millions of times larger, which ought to be fet in oppofition to the fpermatic animalcula, in order to draw a conclusion in favour of the latter.

What has been faid, in my opinion, fufficiently fhews, that even though fertile femen thould alone be always full of animalcula, it would by no means follow, that they are the cause of the fertility of that fubftance. As to the refemblance, faid to be found between the human spermatic worm and the foetus, the head in both being large in proportion to the rest of the body, I do not fee that any great advantage can be made of it. It is not a very conclufive argument to fay, the animalcule has a large head, the foetus has a large head, ergo, the animalcule makes the fœtus. From the way in which this animalcule is figured, neither its body nor its head, have any refemblance, in external form, to the head and body of the fœtus. It rather refembles a tadpole, confifting of nothing but a head and a

tail. Mr Andri himfelf makes this comparifon. Now, as what is taken for the head of the tadpole, is in reality its whole body, inclofed in an orbicular fpace; may not the fame be the cafe with the fpermatic worm, and then, what becomes of its resemblance to the foetus ?

We conclude, from all this, that the fyftem of Leewenboek is not built on any folid foundation: and that therefore, even though it should not be attended with difficul ties, we ought never to confider it, but as a mere conjecture, which we may admit or reject, as we find convenient, and of which, a little more or lefs 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 following 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 infects is full of animalcules, as well as that of other animals. He has been able to discover them in that of beetles, dragon-flies, grafshoppers, gnats, and even in fleas; but Swammerdam, who is not used to advance facts on flight grounds, lays it down as certain, that the foetus of infects, from the very formation of the egg, and confequently, long before copulation, fills the whole capacity of the egg. If this be true, it must follow, of neceffity, that this fœtus 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 foetus formed, without the affistance of fpermatic worms, and that, even in an animal which poffeffes them. Is any thing more needed to overturn the fyftem I am examining?

I obferve, in the fecond place, that Leewenhoek, in a letter, without date, written to Sir Chriftopher Wren, and inferted in the collection of his letters, printed in 1696, fays pofitively, that he had found two forts of fpermatic worms in the fame fubject, whence he concludes, that the one produces the male, and the other the female.

But, have we not better reafon to conclude, that they produce neither? Indeed, if these animalcula differed from one another, only in fex, is it probable that the difference would

would be fo great, in animals of fuch extreme minutenefs, as to make them appear animals of two different species? And, if they really are animals of two different fpecies at first, how can they afterwards become animals of the fame fpecies, differing only in fex?

My third obfervation refpects the origin of thofe fmall animals. They are not found, according to Leewenhoek, and those who adopt his fyftem, in early youth; at the age of puberty, their number is prodigious; they almost all perifh in difeafe; they appear again, on the return of health, and the vast quantity,loft at the union of the fexes, is always fupplied while the generative faculty remains. From all thefe facts, we cannot but conclude, that thefe 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 muft allow, that there refides in the feminal matter, or in the veffels which form it, a faculty, capable of producing daily, hundreds of millions of living beings, without the affiftance of any animalcule; and if fo, why may it not be allowed, that the fœtus can be produced in the fame manner, without the fame affiftance, by a fimilar faculty? But, if it is maintained, that the animalcules in queftion are multiplied in the fpermatic 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 faid to have begun to be animals; but, according to the principles of Leewenhoek, we muft alfo allow, that in their femen, there are other animals, infinitely fmaller, to which they owe their origin, as thefe other animals muft, in their turn,owe theirs to animalcules, fmaller ftill, in the fame proportion, which might be carried ad infinitum, unlefs we should find at last some, whose femen had the faculty of fecundating the female, without the affiftance of fmall animated pre-cxifting beings. And, if we must come to that at laft, what do we gain by the system of Lecwenhoek? And, why not allow the fame faculty to the femen of larger animals?

In the fourth place, if it is maintained, that the fœtus is formed of one of these fmall fpermatic animalcules, we muft fuppofe it to grow, with fuch furprising rapidity, as, if it is not altogether impoffible, is at leaft incredible, and has

no