94 THE COURTSHIP OF ANIMALS 



which they have aroused will be the more readily 

 appreciated. 



Definitions are always liable to exceptions ; and con- 

 crete cases are better than abstract terms. Birds, then, 

 perhaps better than any other group, illustrate what is 

 meant by the term " secondary sexual characters," 

 if only because examples are so constantly at hand. Save 

 among experts, sex among birds cannot be determined 

 except by the differences in plumage, or sometimes in 

 size, which the sexes display. But even here, it is only 

 among species which occupy what we may call a mid- 

 evolutionary phase in which this discrimination is 

 possible. Among " generalized " species, wherein the 

 plumage is of sombre hue, there is no external dis- 

 tinguishing mark between male and female ; and the 

 same is true with species which have attained to the 

 maximum of resplendent plumage ; as for example many 

 Parrots and Kingfishers, where again both sexes, and at 

 all ages, display the same vivid hues. Thus, in the case 

 of either of the two extremes, the study of behaviour 

 during the breeding season is one of great difficulty and 

 no less uncertainty. Where the sexes are sharply dis- 

 tinguished by differences of coloration, however, as with 

 the Peacock, the matter is otherwise. This bird, from 

 time immemorial the symbol of vanity, illustrates in a 

 singularly effective manner the broad features of what 

 is commonly meant by " courtship " among birds, while 

 it furnishes a no less striking example of the development 

 of " secondary sexual characters." 



One might have supposed that birds, under the spell 

 of that irresistible desire for sexual intercourse, would 

 behave differently in regard to their " courtship " accord- 

 ing to whether they were monogamous, polygamous, oi 



