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Rh primary constituents. And if this be so in these instances we are not, it appears to me, justified in doubting that the eggs of sexually dimorphic animals in general contain double primary constituents, though these are not distributed in two distinct kinds of eggs, but are contained in the same germ-plasm. From this latter condition we pass readily to the relations that exist among the social insects, the germ of which must contain at least three sorts of primary constituents of the body, inasmuch as those of the female occur in two forms.

Let us now inquire how it is possible for the degeneration of the primary constituents of the ovary to come about. It is clear that this could not be a consequence of disuse, as infertility ceases to be transmitted in proportion as it increases. It therefore seems to me that the relative disappearance of egg-tubes in the workers of bees and ants furnishes a convincing proof that it is a mistake to regard the degeneration of any organ as a direct consequence of disuse. In the case we have been considering the very organs on which transmission depends have degenerated, and accordingly the degeneration could not be transmitted at all. The degeneration of the ovary has nevertheless proceeded on its slow phyletic course, step by step, and has caused one egg-tube after another to disappear; just as organs that are no longer used—such as the supernumerary toes of the ancestral horse—have to all appearance degenerated through the direct effects of disuse, their reduction being transmitted to subsequent generations. But in the case we were here dealing