282 ANIMAL COLOKATIOX. 



If, on the other hand, there is generally this correlation -if, 

 as has been here argued, ornament is the natural product and 

 direct outcome of superabundant health and vigour, then no 

 other mode of selection is needed to account for the presence 

 of such ornament. The action of natural selection does not 

 indeed disprove the existence of female selection of ornament 

 as ornament, but it renders it entirely ineffective." 



Summary. 



In short, we find that the secondary sexual characters of 

 animals are dependent upon the germ glands themselves ; 

 and that the sexual diversity of animals is also associated 

 with differences of disposition and habit. There is a funda- 

 mental difference between males and females, based upon the 

 actual difference of sex, which generally finds an expression 

 in outward unlikeuess. These superficial differences may also 

 be partly due to the different mode of life led by the two 

 sexes. We meet with them in animals which cannot be 

 moved by any choice or aesthetic preference : but it is also 

 true that they are most highly developed in the higher animals, 

 where such choice is at least conceivable ; the mammal, how- 

 ever, forms a very important exception to this statement. 

 Butterflies and birds show the most marked sexual dimorphism 

 in colour ; and it is precisely in these two groups that there 

 is the greatest opportunity for colour development, owing to 

 the structure of their feathers and scales respectively. Colour 

 differences become necessarily exaggerated in these animals, 

 through mere multiplication of details. Nevertheless, it is 

 quite possible that sexual selection may have played a subordi- 

 nate part in the production of sexual coloration, and we may 

 also allow some force to Stolzmann's suggestions. 



