PAPERS ON ZOOLOGY 281 



sexes. Such characters are teiTued secondary sexual 

 characters. 



Much of the literature concerned with the explanations 

 of dimorphism has placed strong emphasis upon the role 

 of sexual selection as the factor which has originated 

 and emphasized the secondary sexual characters. Most 

 of the popularly cited instances offer no fundamental 

 difficulty for such a possible origin. The development of 

 horns or other organs of offense or defense in the males 

 of mammals would seem to give such individuals greater 

 chance of perpetuating their kind than possessed by 

 other individuals not having such special organs. Simi- 

 larly, there does not seem to be any fundamental objec- 

 tion to the possibility that females among the birds 

 might show preference for the more highly colored and 

 ornamented males as mates though many authorities 

 question any such show of preference on the part of the 

 female. In all of the most readily available accounts 

 sexual selection and natural selection have been almost 

 exclusively advanced as basis for the explanation of di- 

 morphism. In fact. P. C. Mitchell in the Encyclopaedia 

 Britannica (Eleventh Edition, Vol. 24:748) definitely 

 contends in his discussion of Sexual Dimorphism that 

 Darwin's theory of sexual selection is the only compre- 

 hensive suggestion capable of explaining why some males 

 and females differ and others resemble each other. 



In species having no direct copulation and in all those 

 having no mating of the sexes, obviously sexual selection 

 cannot operate in the development of secondary sexual 

 differences. Frequently ardent advocates of a theory 

 have been so blinded by the implicit belief in the all 

 potent powers of some particular explanation of given 

 phenomena that they have been unable to conceive of 

 the possibility that various factors may act simultan- 

 eously to attain the same or similar end results. This 

 seems to have been the attitude of recent writers who 

 have tried to explain all secondary sexual characters on 

 the basis of natural and sexual selection. Charles Dar- 

 win, the founder of the theory of sexual selection, has 

 weU pointed out that his theory could not be the sole 



