Among insects, some (_e.g._ grasshopper, cricket, ant, etc.) produce young in the ordinary way, by the union of the sexes; in other cases (_e.g._ flies and fleas) this union of the sexes results in the production of a _skolex_; while others have no parents, nor do they have congress--such are the ephemera, tipula, and the like. Aristotle discusses and rejects the theory that the male reproductive element is derived from every part of the body. He concludes that "instead of saying that it comes _from_ all parts of the body, we should say that it goes _to_ them. It is not the nutrient fluid, but that which is _left over_, which is secreted. Hence the larger animals have fewer young than the smaller, for by them the consumption of nutrient material will be larger and the secretion less. Another point to be noticed is, that the nutrient fluid is universally distributed through the body, but each secretion has its separate organ.... It is thus intelligible why children resemble their parents, since that which makes all the parts of the body, resembles that which is left over as secretion: thus the hand, or the face, or the whole animal pre-exists in the sperm, though in an undifferentiated state ({ adioristôs}); and what each of these is in actuality ({ energeia}), such is the sperm in potentiality ({ dynamei})."
In later times the two great rival theories put forward to account for the development of the embryo have been--
(_a_) The theory of Evolution, which makes the embryo pre-existent in the germ, and only rendered visible by the unfolding and expansion of its organs.
(_b_) The theory of Epigenesis, which makes the embryo arise, by a series of successive differentiations, from a simple homogeneous mass into a complex heterogeneous organism.
The above quotation will show how closely Aristotle held to the theory of Epigenesis; and in another place he says, "Not at once is the animal a man or a horse, for the end is last attained; and the specific form is the end of each development."
Spontaneous generation is nowadays rejected by science; but Aristotle went so far as to believe that insects, molluscs, and even eels, were spontaneously generated. It is, however, noteworthy, in view of modern investigations, that he looked upon _putrefying_ matter as the source of such development.