amount of their food; and similar results have been obtained by several observers with regard to certain caterpillars, in which, as in tadpoles, the males appear in greater numbers when nourishment is scarce. Experiments made by Maupas, on the other hand, show that in Hydatina senta, one of the fresh-water rotifers, the determination is effected by the temperature; and this instance is specially interesting, as the sex of the offspring is decided even before their primary constituents are formed in the germ (see Note VII, p. 60).
Siebold and Leuckart have shown that in bees and wasps the eggs which are fertilized develop into females, while those that are not fertilized give rise to males. And though we are still completely in the dark as to how this occurs, its utility at any rate is manifest, for the queen is thus enabled to produce male or female offspring at will or according to necessity. To this extent we can understand why the sex has here been made to depend on an external impulse; and probably no one would in this case conclude that the stimulus is the efficient cause of the male or female character of the embryo: no one regards the warmth necessary for the development of a pigeon's egg as the cause of its giving rise to a pigeon and not to a duck (see Note VIII, p. 60).
In other instances, however, this is less apparent, and the stimulus is readily mistaken for the causa efficiens of the development. Permit me to go somewhat more into detail with regard to one such case which seems to me of peculiar interest; and which, although generally known, has never yet been
