EVOLUTION OF THE COLORS OF BIRDS. 213 



character; or again, they may differ greatly from the 

 adults in both their seasonal plumages. 



VI. In some few cases the young in their first plu- 

 mage differ from each other according to sex; the young 

 males resembling more or less closely the adult males, 

 and the young females more or less closely the adult 

 females." 



To these classes as given by Darwin the following may 

 be added: 



VII. When the adult male is unlike the adult female, 

 the young of both sexes .may differ from either sex of 

 the adult, as with the blue-bird (Sialia sialis). 



VIII. When the adult male is unlike the adult 

 female, the young male sometimes has a peculiar first 

 plumage, while the plumage of the young female is like 

 that of the adult female, as in many of the woodpeckers. 



These classes maybe more clearly presented in a table 

 commencing with the most developed forms in which 

 the characters originating first have been transferred 

 first to the female and secondly to the young, and 

 proceeding down from this point through the classes 

 where the inheritance is less and less complete. Dar- 

 win has called attention to the fact that these various 

 divisions are not sharply marked, but shade into one 

 another, so that in many instances it is difficult to say 

 definitely to which class a particular species should be 

 assigned. It should also be remembered that while in 

 a large number of instances sexual selection has been the 

 factor which has produced the difference, it is not 

 always so, sexual intensification, sexual recognition and 

 other factors having played a part. 



