passive, the work just quoted abundantly proves. Now when neuters are produced from males their duties differ from the neuters produced from females, and, as we have supposed that the duties now performed by sterile females were once performed by fertile females, so we may now conclude that the duties performed by sterile males were once performed by the perfect males. The worth of this conclusion will be presently seen.
The remaining difficulty is to account for the fact that with many insects the neuters differ considerably from the fertile insects. Thus, for example, with the termites or white ants; these are the perfect males and females, the soldiers, which are aborted males, and the workers, which are aborted females. The males and females have wings, the neuters are wingless; the workers undertake architectural duties, act the part of nurses, etc., while the soldiers defend the nest from attacks. Both workers and soldiers are blind, but whereas the workers have a somewhat circular head and small jaws, the soldiers have a comparatively enormous head, and strong resisting mandibles. In what manner then, or through what cause, could the head of this soldier termite differ so greatly from either that of the perfect male or perfect female? Or, since it is impossible in any given case to explain all the details satisfactorily, let it be asked how it is that so many neuter insects differ from their parents.
Neuters are either sterile males or sterile females, and in many cases do not differ greatly from their fertile progenitors; the social bees and wasps are examples of this. On the theory advanced, it has been supposed that originally the neuter differed only from the perfect insect in that it had a rudimentary reproductive system. Now, suppose, to take an imaginary example, that in a colony of ants there are only males and females; that the duty of the male is the defense of the nest against encroaching enemies, and that the duties of the females are to build the nest, lay eggs, and take charge of the young. Of course the males and females having different duties to perform will have their structures differently modified; say, in our case, the male has a largely developed head like the soldier-termite, the female a head like the worker-termite. Going a step further, neuters begin to appear, the aborted male still performing soldier duty, the aborted female still attending to its domestic duties. The neuters continuing to increase as we know they have increased, and the true males and females decreasing in number as we know they have decreased, a state of affairs is reached in which it is essential to the welfare of the colony that the male should confine himself to fertilizing the female, the female principally confine herself to laying eggs.
Thus far, the fertile and infertile males, the fertile and infertile females, have resembled each other; but disuse of parts induces retrograde metamorphosis, or modification or suppression of useless parts. If the males no longer use their heads and jaws to protect the com-
