This discussion leads to the question of the natural process of generation itself. It is a topic that we have seen discussed by an earlier writer who had set forth a sort of doctrine of pangenesis (see p. 14). His view Aristotle declines to share. ‘We must’, he says, ‘say the opposite of what the ancients said. For whereas they said that semen is that which _comes from all_ the body, we shall say that it is that whose nature is to _go to all_ of it, and what they thought a waste-product seems rather to be a secretion.’ According to Aristotle semen is derived from the same nutritive material in the blood vessels that is distributed to the rest of the body. The semen, however, is strained or secreted off from this nutritive material—as being its most essential and representative portion—before the distribution actually takes place.[31] But why, it may be asked, if the semen does not come from the various parts of the body, is it yet able to reproduce those various parts? The answer, on the Aristotelian view, seems to be that the semen contains special and peculiar fractions of the nutritive fluid which have been so modified and adapted that, if not secreted off as semen, they would be distributed to the different parts of the body to nourish each of these various parts. These substances have been elaborated by the _soul_ or vital principle in a manner that is specifically suited for each organ, hand, liver, face, heart, &c., and from each of these specific substances a specific essence is separated off into the semen corresponding to hand, liver, face, heart, &c., of the offspring. The next question that arises is the mechanism by which the offspring come to resemble their parents. The mechanism in the case of inheritance from the father is comprehensible when we consider the origin and nature of the semen, but the inheritance from the mother requires further explanation. The view of Aristotle is based upon the nature of the catamenia and their disappearance during gestation. ‘The catamenia’, in his view, ‘are a secretion as the semen is.’[32] The female contributes the material by which the embryo grows and she does this through the catamenia which are suspended during gestation for this very purpose. The matter is thus summed up by Aristotle. [31] _De generations animalium_, i. 18; 725ᵅ 22. [32] _De generatione animalium_, i. 19; 727ᵅ 31.