2o6 THE BIOLOGY OF BIRDS 



labour between the two parents in their relations to the off- 

 spring ; and so on. But at a great distance the sex- 

 divergence attained another justification ; it became the 

 basis of love, it served as a liberator and educator of emotions 

 which have enriched and ennobled the lives of many 

 creatures. As Darwin clearly recognised, characters which 

 were primarily selected in relation to mating might become 

 secondarily diverted by a function-change to even wider 

 issues. 



In his " Studies in Animal Behaviour " (1916) Mr. S. J. 

 Holmes has an interesting chapter on the role of sex in 

 the evolution of mind. " The primary function of the vocal 

 apparatus of the vertebrates was probably to furnish a sex- 

 call, as is now its exclusive function in the Amphibia. Only 

 later and secondarily did the voice come to be employed in 

 protecting and fostering the young, and as a means of social 

 communication. And the evolution of the voice in verte- 

 brates doubtless influenced to a marked degree the evolution 

 of the sense of hearing. It is not improbable, therefore, 

 that the evolution of the voice, with all its tremendous 

 consequences in regard to the evolution of mind, is an out- 

 growth of the differentiation of sex." There can be little 

 doubt that the biology of the future will attach not less but 

 more importance to sexual selection. For it seems likely 

 that characters and qualities originally established in this 

 way have often influenced both body and behaviour in 

 reaches now more or less remote from the tides of sex- 

 impulses. 



§ II. Sex- Characters 



Since the beginning of the twentieth century the difficult 

 problem of the origin, evolution and development of sex- 

 characters has been illumined by a series of brilliant ex- 

 perimental researches, which have made reconsideration 

 imperative. This has been facilitated by the masterly work of 

 Kammerer (" Ursprung der Geschlechtsunterschiede,"i9i2). 



A. Sex Differences. — It is usual to classify the differences 

 between the sexes as " primary " and " secondary." The 



