ftill longer. This animal according to various authors is folitary, and, as they pretend, is formed in the human foetus before it is born: it grows up with the person, and there is never more than one found in the body it inhabits. If these circumstances be true, as Hippocrates and his followers affert, what are we to believe concerning the origin of fuch an animal? For there have never been found, out of the bodies of animals, any thing fimilar to them, from which we might prefume they were derived and if there had, either great or fmall, their flat, thin figure, and the vast multitude of articulations they are composed of, would certainly have made them be taken notice of. It must therefore be admitted that these worms are only produced from thofe that are found in animal bodies; and if fo, how could they be generated but on the fuppofition that each fingly is fufficient of itself to produce its like, being always found folitary? and thus we have an inftance of our hermaphrodites in the Tænia.

I know that this fuppofition does not remove all the dif ficulties attending the origin of this fingular worm. It may still be asked why it is always found fingle, and by what means its eggs or its young enter the body of another perfon? But it would not be difficult to answer thefe questions by new fuppofitions. The first disappears by fuppofing that this worm is among the number of those that devour one another, the strongest, having eaten up all thofe produced along with it in the fame place, muft neceffarily remain alone. As to the other difficulty we have only to fuppofe that the egg or the foetus of this worm is extremely small, and that the animal depofits it in our chyle, which it may do eafily, if the orifice of its ovary is fituated near its head, as that of the fnail is. From the chyle it will enter into the mafs of blood: if it inhabits a woman, the communication her blood has with the foetus in her womb will introduce the egg or the fœtus of the worm into it by means of the circulation, and the egg and foetus will begin to grow as foon as it arrives at the placé destined for its habitation. If it is harboured in the inteftines of a man, then the egg entring the mafs of blood, will be carried by the circulation into the veffels where that blood is elaborated, and prepar ed for a purpofe neceffary to the prefervation of our fpecics; and thus we fee eafily how it may be mingled with the particles, which enter into the compofition of the human fœ

tus

tus. It is thus that, proceeding on fuppofition, it is easy to account for every phenomenon, even for the existence of things which have never exifted, as thofe philofophers have done, who have explained to us the way in which putrefaction engenders infects. I have perhaps at prefent imitated them, by founding, in the inftance of the Tania folium, on facts, which, though pretty generally received are not on that account the more worthy of credit. I know at least that Vallifneri, has endeavoured to render them doubtful, and to maintain that the Tænia is nothing but a chain of worms called Vermes Cucurbitini, which are linked to one another, and thus form aggregately the figure of a fingle animal. The reafons he alledges have a good deal of plaufibility, and feem fo ftrong that at prefent a perfon would be accused of prejudice not to fubfcribe to his opinion. But I must own they have not entirely convinced me. The difficulties which have occurred to me on the fubject, will induce me to neglect no opportunity of endeavouring to difcover the truth; and till I have examined the animal alive, which I fhall do if I can procure it, I know not whether I fhall adopt the opinion of that learned author, or continue of the contrary fide.

What has been faid fufficiently fhews that though it is probable that there may be infects which multiply naturally without the common process by which generation is accomplished, the fact is not yet demonftrated. But what may be advanced as a certain fact, though it has ftill more the appearance of a paradox is, that there are animals which can be made to multiply, and which in fact do multiply by art, without the common generative process, as we fhall have occafion to fhew in the fequel.

PAGE 30, l. 14,

Engendered by all forts of matter. Ariftot. Hift. Anim. L. 5. c. 19, Procreantur porro infecta, aut ex animalibus generis ejufdem - - - aut non ex animalibus, fed fponte; alia ex rore qui frondibus infudat; item alia ex cæno et fimo putrefcente oriuntur: alia in lignis aut ftirpium, aut cæfis: alia in animalium pilis, alia in excrementis, aut jam excretis aut adhuc intra animal contentis. Add. Plin. Hift. Nat. L. 11, cap. 33.

PAGE 31, 1. 31,

The covered veffel did not contain any animals. To the exexperiments

periments of Rhedi, may be opposed that made by Leewen hoek, which he relates in his letter of the 14th of July, 1680. He there fays, that he had heard various opinions on the generation of infects: that he had even learnt, that a certain author had maintained, that, if a veffel, containing water and flesh, were carefully fhut, no animal could be produced in it: that this had led him to try the experiment ; that, having taken for this purpose, two tubes of glass, shut at the bottom, he had filled them half full of pepper, and had poured on water, fo as to fill the glaffes about threefourths. It was rain water, recently fallen, and was received into a porcelaine jar, very clean, and which had not been made ufe of for ten years: that having hermetically fealed the top of one of thefe glaffes, and having left only a fmall opening in the other, he examined the water three days afterwards in the open tube, and discovered in it, a great number of very minute animalcules of different kinds, moving in all directions: that having, on the fifth day, broken off the end of the sealed tube, the air iffued with violence, and he difcovered in the water of this tube, a fpecies of globular animals, larger than the largest of those in the other tube. Here, then, were animals generated, in a place clofely fhut, and where no infects could enter to depofite their eggs: which appears quite contrary to the experiments of Rhedi, and furnishes an argument in favour of equivocal generation. But, if we attend to the animals which make the fubject of thefe experiments, the difficulty will foon be removed. It is certain, that the experiments of Rhedi were made on those maggots that are of a fenfible bulk, and which, without the aid of a magnifier, are every day feen in putrid animal fubftances. His object was to prove, contrary to the opinion of the antients, that these maggots were not produced by the corruption of the animal matter, but fprung from eggs which flies had laid in it; and this appeared clearly from the precautions he took to keep off the flies. He contented himself, with covering the mouth of the veffel with a thin cloth, a precaution which would have been ufelefs against infects, incomparably more minute, but which was fufficient to exclude common flies.

The experiment of Leewenhoek, on the contrary, refpects animals of a quite different kind, animals, of which a vaft number may live in a fmall quantity of water: animals which he

calls

calls very minute, that is, in his ordinary ftile, animals, that it would require a million of, nay ten millions, and fometimes an hundred millions, to compofe the bulk of a grain of fand in a word, animals which one would not fuppofe a microfcope could make visible, had he not taken care to demonstrate its poffibility. We cafily fee, that the precauti ons which Leewenhoek took, to exclude fuch animals from the tube he had fealed, were by no means fufficient. Thefe a❤ nimals or their eggs, might have been, either among the pepper,or in the rain water he employed,or even in the air which filled the void in his tube: there was, therefore, nothing furprifing, in animals being found there, five days afterwards. To overturn, by his experiment, what had been proved by Redi's, Leuwenhoek ought, at leaft to have boiled the wa ter and the pepper in the very tube, and then to have fealed it immediately. If he had then found, fome days afterwards, any animals in that water, his experiment would very much have difconcerted the modern naturalifts: but, I ain perfuaded, that is what would not have happened.

PAGE 36, 1. 29.

From which fimilar infects are to be generated. This ingeious comparifon, which fhews the conformity of infects with plants, is fimilar to that made by Swammerdam, in the firft part of his General Hiftory, where he compares the developement of the different orders of infects with the plant called the carnation. The greater animals may, in fome refpects, be likewise brought into the comparison of M Leffer, fince all, or at least, many of them, proceed likewife from an egg; that all of them increase, by means of a nutritive fluid; and that in general, they do not propagate their kind, till they have attained their ultimate perfection. It muft, however, be confeffed, that fome of the analogies which our author difcovers between infects and plants, are but imperfect. That, for instance, of the wings of infects with the leaves, feems a little far fetched; for, in the first place, the leaves appear, almoft as foon as the bud begins to open, while the wings of infects never make their appearance, till they have attained their perfect ftate. Secondly, The leaves grow flowly after being difengaged from their gems; but the wings of infects, after having quitted their covers, elongate themfelves immediately, and acquire their full fize in a few minutes. Thirdly, the number of leaves in a plant is not fixed; they fall and are renewed, and this

Pp

viciffitude

1

viciffitude lafts as long as the plant itfelf: but the number of wings in every different infect is invariable, and a wing once loft, can never be fupplied. Laftly, according to the conjectures of the moft able botanifts, the leaves are given to plants, to defend the root and the ftem from the heat of the Sun,to facilitate the evaporation of fuperfluous humidity, and to promote the circulation of the fap, which is to elaborate and prepare the fhoots, the fruit and the feeds; while the wings are beftowed on infects, for a very different purpose, to wit, to facilitate their motion from place to place. Be. fides, if the wings of infects in general, refembled what is faid of thofe of a certain Indian infect, called in that coun try, the walking leaf, (Cicada foliacea; Gryllus ficcifolius,) their analogy with the leaves of plants, or at least of trees, would be more remarkable. The wings of thefe infects, not only in the form and nerves of the wings, resemble the leaves of trees, but in their colour. I have seen some of them with wings of a bright green, others, with wings of a darker green, and like that of a leaf at full growth, and o thers of the colour of a withered leaf. It is faid, that their wings are of the first colour in fpring, of the second in fummer, and of the laft in autumn; that afterwards, they fall, and that the infect remains without wings during the winter, and that they fhoot forth again in the fpring. If all these circumstances were true, we could not deny, but that the wings of this infect had a very marked analogy with the leaves of trees; but, at the fame time, we must allow, that in this respect, it differs from other infects, and is probably unique in its kind; at least, there is not, so far as I know, any other, whofe wings are liable to the fame viciffitude.

In fine, it may be obferved, that the comparison made by the author, between a nymph or chryfalis, from which a perfect animal iffues, and a flower-bud, which produces fruit in its maturity, exceeds fomewhat, the terms of the parallel in queftion. His object was to fhew the analogy between infects and plants. For this purpose, the author compares the egg of an infect to the feed of a plant, its body to the ftem, and its wings to the leaves. It would be neceffary, in order to compleat the analogy, to compare fome other part of the infect with the flower-bud, but not to compare with it, the entire infect, as he has here done,

PAGE 37, 1. 4.

On which they are placed, If Mr Leffer was content, in

this