NATURAL SELECTION: II 491 



Muller (1949), without risking the fate of the entire species on the outcome 

 of one experiment. 



SEXUAL SELECTION 



Darwin considered that the development of the so- 

 called secondary sex characteristics could not be adequately explained by 

 his theory of natural selection. The primary sex characteristics are, of 

 course, the male and female reproductive organs. But in addition to the 

 latter, males usually differ from females in a variety of bodily structures. 

 The bright plumages and elaborate songs of many male birds, contrasted 

 with the duller plumages and relative tunelessness of the females, form a 

 familiar example. Among mammals it is the male lion that develops a 

 mane, the male goat that possesses a beard, the male deer that displays 

 many-pronged antlers. Darwin felt that such differences between the sexes 

 are not vital enough to the welfare of the species to arise through the opera- 

 tion of natural selection. Hence, he advanced the supplemental theory of 

 sexual selection to explain their development. 



Although one would not expect it from the abbreviated title by which 

 the book is usually known, the theory is set forth in detail in Darwin's book 

 The Descent oj Man. Bodily characteristics with which the theory is 

 concerned may be divided into two main types: ( 1 ) those of use in combat 

 between rival males and (2) those used for display purposes. Both types 

 might be useful under conditions of competition between males for 

 mates. 



Male birds or mammals are sometimes observed to fight for possession of 

 a particular female, although the number of species in which this struggle 

 occurs has probably been much overestimated in the past. When such com- 

 bats do occur, males with superior implements of warfare might be ex- 

 pected to be most successful, becoming the fathers of a disproportionate 

 share of the next generation. The male offspring of such fathers might be 

 expected to inherit their fathers' superior fighting equipment, and if the 

 process continued long enough males of that species might become quite 

 unlike the females in terms of bodily equipment specifically connected with 

 fighting. 



But how about the development of features concerned with display — 

 bright colors, ornate plumages, songs, posturing, dancing, and the like? 

 The theory accounts for their development by assuming that females pos- 

 sess aesthetic sense and employ it in deciding which suitor to accept. Thus 



