202 THE POPULAR SCIENCE MONTHLY 



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 mat- 

 ing, 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 articu- 

 late language. It is not improbable, therefore, that the evolution of the 

 voice, with all its tremendous consequences with respect to the evolu- 

 tion 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 dioecious 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. 



