greatly the institution of sex has stimulated the evolution of complex modes of behavior.
All the facts here cited are trite enough, even to the non-biological reader. But while it is sufficiently evident that the differentiation of the sexes has promoted the development of behavior in relation to mating, it may be well to point out the enormous indirect consequences of this development in respect to the evolution of mind in general. In the evolution of behavior one kind of instinct grows out of another just as new organs are usually formed through the elaboration of some pre-existing structure. A general elaboration of instinctive reactions in regard to any one sphere of activity affords, therefore, a basis for the differentiation of more complex and specialized behavior in respect to other activities. To take a concrete illustration: The primary function of the voice in the vertebrates was to serve as a sex call, as is now its exclusive function among the Amphibia. Later and secondarily it came to be employed in relation to the protection of the young (through various instinctive calls) and as a means of communication with other members of the species. Finally in man it afforded the means of articulate language. It is not improbable, therefore, that the evolution of the voice, with all its tremendous consequences with respect to the evolution of mind, is an outgrowth of the differentiation of sex. Were it not for its value in effecting the mating of the lower vertebrates the voice might never have been evolved and man never have become man.
While the specialization of senses which in certain cases at least has been carried on mainly for sexual purposes has doubtless afforded the basis for the elaboration of many instincts, it is practically impossible to trace in detail how various instincts, sexual and other, have acted and reacted on one another's development. But we can discern enough of the influence of sex differentiation on the evolution of behavior to feel assured of its importance. The necessity for solving the one problem that confronts all diœcious animals which do not simply shed their sexual products at random into the water, has tended to keep behavior in one sphere up to a certain minimum standard. The male must find and impregnate the female, and this fact sets a certain limit to his degeneration, at least in some period of his life, because any further degeneration would involve fundamental changes in the method of reproduction which may not be possible. But besides acting as a check to degeneration, the necessity for mating has in general been a constant force making for the evolution of activity, enterprise, acuity of sense, prowess in battle, and the higher psychic powers. One can not pretend, except in the most general terms, to gauge its role in the evolution of mind, but it has evidently been a factor of enormous potency.
