SECONDARY SEXUAL DIFFERENCES 201 



males. It would be useless foi* a more highly colored 

 male to attempt to obtain possession of a mate if a 

 less highly colored but more vigorous male were pres- 

 ent to dispute the possession. 



Mr. Darwin says: "It is evident that the brilliant 

 colors, top-knots, fine plumes, etc., of many male 

 birds cannot have been acquired as a protection ; 

 indeed, they sometimes lead to danger. That they 

 are not due to the direct and definite action of the 

 conditions of life, we may feel assured, because the 

 females have been exposed to the same conditions, 

 and yet often differ from the males to an extreme 

 degree."* 



He thinks that "it is possible that at first there 

 existed a tendency to transmit the successive varia- 

 tions equally to both sexes; and that the females were 

 prevented from acquiring the bright colors of the 

 males, on account of the danger to which they would 

 have been exposed during incubation." 



But, as already stated, the bright colors would also 

 subject the males to greater danger, and this being 

 true, it would have prevented the males from becom- 

 ing ornamented. The arguments that apply to the 

 one sex in this respect apply to the other. The nat- 

 ural tendency of a variation would be to change both 

 sexes alike. The many differences between the sexes 

 of the higher invertebrates and of the majority of 

 vertebrates must be accounted for on certain general 

 principles if the doctrine of evolution is true. 



If it were granted that sexual selection might be 

 more or less efficient in producing changes among 

 animals so highly organized as birds, yet this could 

 not be a sufficient reason for extending the principle 

 to the lower vertebrates and to the invertebrates 

 where sexual differences prevail. 



* Descent of Man, Vol. 2, p. 224. 



