網頁圖片
PDF
ePub 版

manner. These productions, so monstrous and so uncouth in appearance, are not on that account the less subject to the constant and general law of nature, that each animal produces its like. If we often see insects of the fame species, proceeding from animals of a very different kind, it is not that the latter have generated the former v but the semales of the one having introduced their eggs into the bodies of the others, the young have been produced from these eggs, and after having sed on the substance of the animal Jn whose body they were inclosed, issue from it in the form of the insects that had placed them there. These are facts now universally known, and which I have verisied by experiments not necessary here to be detailed.

Page 28, 1. 33.

They cannot repair the loss. This appears so certain, and is so consormable to the idens we have of the fornufion of organised bodies, that one would not expect to sind exceptions to the rule. But the author of nature, whose wisdom consounds all our reasoning, in order, as it would seem, to shew us how litrle we ought to depend on our own power* when we judge of his operations, has created animals which form a very remarkable exception to it, having the singular faculty of reproducing their members whenever they are deprived of them. Sea stars, crabs and lobsters are instances of this, which cannot now be doubted, after what has been related by a naturalist of superior abilities, in the Memoirs* of the Royal Academy of Sciences, for the year 1712.

But these instances, and others which I might mention-, by no means affect Mr Lesser's reasoning. It is not the sea star, the crab or the lobster that replace the limb they have' lost; it is nature which gives it to them, and they contribute as little to the reproduction as we do to that ot our nails or our hair.

Page 30, 1. 3.

They multiply by generation. It is a general law of nature" that animals preserve their species, and multiply in the way of generation. This has never been doubted as to the larger aniroah, and when insects have been narrowly examined, it has been found that even they whose production seemed the most equivocal, owe likewise their origin to the union of a male and semale of the same species. But however general this rule mav be, the universality os it is not

'yet yet established. The various ways in which propagation is seen to take place among insects, seem to authorize a doubt on this subject. There are several sorts of those animals formerly classed among insects, such as snails and earth-worms, which are both male and female in one individual. We find among true infects certain species, the greater part of which are neither male nor female, as bees, wasps and ants. Some are observed to engender withouwcoupling, the male contenting himself with depositing his semen on the eggs of the female, as in the Ephemerae. Some are found which can produce a posterity for several generations by a single embrace, as I have discovered to be the case with the aphides. If we believe SwammerJam on this head, who however gives no solid proof of his opinion, there are some females among insects which can be impregnated by the mere smell of the {bale. All these various methods of propagation, lead us to presume that there may be insects which multiply without reciprocal intercourse, and without the process of generation strictly so called, and Where a single individual, by the exertion of its own powers, is sufficient to propagate its kind; but hitherto no author as far as I know, has demonstrated the fact by a conclusive experiment. It is true that Leewenhoeck and Cestoni thought they had discovered such a one among the aphides. Neither they, nor M. de Reaumur ever saw them coupling, or could discover a male one among the whole species; all they examined whether winged or not, were always females, and with young even before they had attained their natural size. These experiments seemed pretry decisive, and others made by me seem still more so. Some aphides, taken away at the moment of their birth, and kept secluded under glasses, in eight or ten days produced young. These young were likewise instantly removed, and bred up in the same solitude, produced others nearly after the fame period, and that production continued long enough to persuade me, by reasons stronger than those of Leewenhoeck or Cestoni, that their opin on was the true one. But having continued my experiments till the season when the leaves began to fall, and not having any farther doubt on the subject, I was all at once undeceived, when I least expected it. I had collected all the young which my solitary aphides had produced, and had established a little colony of them on the extremity of the bran.:h of a willow whicii I kept fresti in a glass of water. The cold had already made tie leaves to wither} but several of the aphides in the O o a 1 nymph nymph state, maintained themselves on it with the rest, and arrived at their persect form. One day as I was visiting them according to custom, I found one of those with wings, sitting on one without wings. I considered this position at sirst as the effect of chance, but the tranquillity of the winged aphis, while the other, disturbed at my approach, was running up and down, made me suspect something. I took a magnifying glass, and upon a near examination, found that rhe. posterior part of the winged insect, bending down towards that of the other, was intimately connected with the under part of it, and exhibited marks os an union in the common form. This attachment lasted more than an hour, after which the winged insect flew away. I saw the fame thing happen to maiiy others of the colony, who formed a "connection like the sirst; and what persuaded me that it Was a true connubial intercourse was, that having accidentally crushed two o'f them in that situation, while I was examining two others, I found after their death, the extremities of their abdomen still attached. The notion therefore of there being animals which can individually propagate their species Is not yet established by experiment made on the aphides; let us see if it is better sounded on those that have been made on the muscles of fresh waters.

Monsieur Mery in the Memoirs of the Royal Academy of Sciences for the year 1710, asserts that it is. He has observed four parts in this animal that may serve for generation : two of these he calls ovaria, because they contain eggs, and two he calls vcstculteseminalcs, because in his opinion they contain the sen: en, which is white, and of the appearance of milk. Their structure appears similar, all four terminate in the anus, where he fays that the two principal ones, as they go out, are united, which is sufsicient tor generation; aud as he has observed in this animal, neither male nor semale organ, he thinks himself the more au:horized to conclude that it is androgynous. But this reasoning, however just it may seem, it is not so conclusive as M. Mery imagines. The parts which characterise the sexes, may bo so disguised by their flexibility, by their situation and form, in an animal whose sigure is so anomalous as the muscle, that it is not impossible but he may have seen them without knowing them; and even though they were actually not to be sound, it would not prove that each muscle was of both sexes. Neither male nor semale organ is to be observed in the greater part of sillies, but are they on that account both male and semale? Besides, though two vessels of the four which terminate in the anus of the muscle, are receptacles of its iggs, it docs not follow that the other twq are the reservoirs of the semen. The milky fluid they contain, may be destined to another use, than that of sructifying the eggs; it may serve to six tht m to the bodies on, which the animal deposits them; to envelope them with a substance which may desend them against the immediate action of the water, to asflrd the young, upon issuing from the egg, a suitable aliment. The eggs ot many aquatic inltcts are surrounded with a clammy matter which they probaoly owe to suc.h vessels. The glue which makes tne eggi of butterflies adhere to the bodies on which we fee them hxe I, proceeds from two vessels terminating in the rectum, and containing a viscid humour, which is any thing but semen; why then should those of muscles contain it? liut e\en tho' they should contain il, would it follow that muldes were individujlly sustic ent for their own multiplication? By no means. The semale butterflies have receptacles which contain semen, and that semen alone is able to secundate their eggs; these receptacles also terminate in the rectum, and inundate the eggs in their passage. But notwithstanding this,these butterflies must enjoy the company of the male, for it is the male alone who surnishes this semen. May this not also be the case w ith the river muscles?

If it were certain that the pholades never leave the hole they form for themselves at the instant of thair birth, as M. de Reaumur maintains, on very plausible grounds ^ Mem. of the Royal Academy of Sciences I "12) one would be tempted to believe that these ihell-lish were sufficient of themselves for procreation, if we did not rather chule to suppose that their impregnation tcok place while they were yet in the matrix of their mother, a circumstance we are not hithci to acquainted with'an instance cf; or perhaps that they have males of another form, and more active than themselves, which visit them in their retreats, as it happens to the gall injects. But if facts lo singular as that in question, could be established by mere reaioning, no animal would, better deserve to be put among the number of thole which are sufficient of themselves to multiply the species than that worm ot the human body called T*triu,t\ie longest perhaps of all animals, since some of them have been sound eighty yards in length, aud it is_not certain but that lome of them may be ftill longer. This animal according to various authors is solitary, and, as they pretend, is formed in the human foetus fcefore it is born: it grows up with the person, and there a never more than one found in the body it inhabits. If these circumstances be true, as Hippocrates and his followers alTon, what are we to believe concerning the origin of tuch an an:mat ? For there have never been found, out of the bodies of animals, any thing similar to them, from which we might presume they were derived: and if there tad, cither great or small, their flat, thin sigure, and the *ast 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 those that are found in animal bodies; and if so, how could they be generated but on the supposition that each singly is sufficient of itself to produce its like, being always found solitary? and thus we have an instance of our hermaphrodites in the Tænia.

I know that this supposition dca not remove all the difficulties attending the origin of this singular worm. It Hiay still be alked why it is always found single, and by what Beans its eggs or its young enter the body of another person? But it would not be difficult to answer these questions ky new suppositions. The sirst disappears by supposing that this worm is among the number of those that devour one another, the strongest, having eaten up all those produced along with it in the same place, must necessarily remain alone. As to the other difficulty we have only to suppose that the egg or the fœtus of this worm is extremely small, and that the animal deposits it in our chyle, which it may do easily, if the orisice of its ovary is situated near its head, as that of the snail is. From the chyle it will enter into the mass of blood: if it inhabits a woman, the communication her blood has with the fœtus in her womb will introduce the egg or the fœtus of the worm into it by means of the circulation, and the egg and fœtus will begin to grow as soon as it anives at the place destined sor its habitation. If it is harboured in the intestines of a man, then the egg eniring the mass of blood, will be carried by the circulation into the vessels where that blood is elaborated, and prepared for a purpose necessary to the preservation of our species; and thus we see easily how it may be mingled with the particles, which enter into the composition of the human fœtus.

« 上一頁繼續 »