The Evidence from Sexual Characters. 169 



species with its nearest allies. The other is by compari- 

 son of the young with the adult. 



If most of the species of a genus resemble each other 

 in certain characters, while one species presents a marked 

 deviation, we may in most cases safely conclude that 

 the latter species has undergone recent modification in 

 this respect. Of course this rule does not hold good 

 where the peculiarities of the exceptional species are 

 features of resemblance to other genera of the family, 

 for in this case we must conclude that it has remained 

 comparatively stationary, while all the other species of 

 the genus have been modified. 



If in the second place we find that the adults of several 

 related species differ greatly, while the young are much 

 alike, we must attribute the difference in the adults to 

 the fact that they have recently diverged from a common 

 stock. 



Now I hope to show that throughout the animal king- 

 dom, wherever the sexes differ from each other, the 

 general law holds good that the males of allied 

 species differ from each other more than the females 

 do, and that the adult male differs more than the 

 adult female from the young. There are many marked 

 exceptions to this law, but the existence of the law 

 has long been recognized by all naturalists. Every 

 one who has worked at the systematic zoology of insects 

 or vertebrates knows how difficult it often is to decide 

 upon the specific identity of an immature or a female 

 specimen, even in cases where the mature males can be 

 recognized and identified without difficulty. 



Darwin's interesting essay on "Sexual Selection" is 

 well known. It is almost entirely devoted to the study 

 of secondary sexual characters, and to a masterly discus- 

 sion of the subject in all its aspects and relations. 



