RELATING TO SECONDARY SEXUAL CHARACTERS. 43 



PART II. 

 DARWIN'S THEORY OF SEXUAL SELECTION. 



Darwin seems to have felt the necessity of giving some other explana- 

 tion for the secondary sexual differences between the male and female 

 than that such differences were only a by-product or concomitant of 

 sex itself. His reason for searching further was probably a part of the 

 general point of view he had reached in regard to the utility of special 

 structures of animals, namely, that their presence finds its explanation 

 on the basis of utility. Believing as he did that most of the adaptations 

 of plants and animals have been built up by the accumulation of small 

 steps, it must have appeared to Darwin inconceivable that the highly 

 developed ornamentation exhibited in the secondary sexual characters 

 could have been simply the by-product of sex itself, especially when 

 the ornamentation may have been entirely absent in males of closely 

 related species. To-day we are not, I think, so oppressed with the 

 difficulties of the situation, for we have become familiar with the fact 

 that very slight genetic differences may cause very great differences 

 in the end-product. In a word, the problem seems less formidable to 

 us than it did to Darwin. 



Darwin appealed to three processes to account for the facts: (1) to 

 natural selection between the members of the same sex; (2) to choice 

 on the part of the " other" sex; (3) to the " inheritance of use." Since 

 each of these appeals to a different procedure, let us take them up 

 separately. 



Competition of the males with each other for the female would, 

 Darwin said, lead to the survival of those males best endowed with 

 organs of offense and defense. The spurs of the cock are weapons dan- 

 gerous for other birds; the horns of the bull and those of deer are used 

 for offense and defense ; the mane of the lion is a protection against the 

 teeth of other lions. It is true that these same weapons and shields 

 serve for attack and defense outside the species; but since the female 

 lacks them or has them less developed, they would not seem necessary 

 for survival of the individual against aggression from without. They 

 have developed, then, through competition within the species. 



Several objections of greater or less weight have been urged against 

 Darwin's interpretation. It has been pointed out that the combats 

 within the species are seldom fatal and that the defeated rival finds 

 another mate. If, as a rule, there are as many females as males within 

 the species and monogamy is the rule, all males will find partners 

 sooner or later, all may have offspring, and the offspring have equally 

 good chances of survival. Under these circumstances it is not to be 

 expected that the combat would be likely to lead to the production of 

 males with longer spurs or larger horns. 



