But it is right here to examine the ideas of Aristotle more generally on the subject of generation, because this great philosopher has written the most on the subject, and treated it the most generally. He distinguishes animals into three classes; first, those which have blood, and, excepting some few, multiply by copulation; the second, those which have no blood, but, being at the same time both male and female, produce of themselves, and without copulation; and thirdly, those bred by putrefaction, which do not owe their origin to parents of any kind. I shall first remark, that this division must not be admitted of; for though in fact all kinds of animals which have blood are composed of males and females, it is not equally true, that animals who have no blood are for the most part male and female in one; for we are only acquainted with the snail and worm on earth which are in this state; nor can we ascertain whether all shell-fish, and other animals which have no blood, be hermaphrodites. With respect to those animals which he says proceed from putrefaction, as he has not enumerated them, many exceptions occur; for most of the kinds which the ancients thought engendered by putrefaction have been discovered by the moderns to be the produce of eggs. After this he makes a second division of animals; those which have the faculty of moving themselves progressively, as walking, flying, swimming, and those which have no such faculty. All animals which can move, and have blood, have sexes; but those which, like oysters, are adherent, or who scarcely move at all, have no sex, and are, in this respect like plants, distinguished only, as he says, into males and females by difference of size. It is not yet ascertained whether shell-fish have sexes or not; there are in the oyster-kind fruitful individuals, and others which are not so; those which are fruitful are distinguished by a delicate border which surrounds the body of the oyster, and they are called males.[BN] [BN] See the observation of M. Deslandes, in the Tracte de la raine, Paris, 1747. But to proceed, the male, according to Aristotle, includes the principle of generative, motion, and the female contains the material parts of generation. The organs which serve for this purpose are different in the different kind of animals; the principal are the testicles in the males, and the matrix in the females. Quadrupeds, birds, and cetaceous animals, have testicles; fish and serpents are deprived of them; but they have both proper conduits to receive and prepare the seed. These essential parts are always double, both in the male and female, and serve in males to stop the motion of the blood, which forms the seed. This he proves by the example of birds, whose testicles swell in the season of their amours, and diminish so greatly when this season is over that they are scarcely perceptible.