350 A HISTORY OF BIRDS 



the secondary sexual characters — the antlers of the stag, the 

 mammary glands of the female, the "breeding plumage" of 

 the bird. Where there are no obviously secondary sexual 

 characters, as in the case of dull-coloured birds, the result is 

 the same — a state of physical exaltation expressed in "dis- 

 play". Males or females wherein these "hormones" are but 

 feebly developed, display and respond indifferently, and so 

 cease to please the opposite sex. As Mr. Howard has pointed 

 out, in the case of the Warblers, no amount of display on the 

 part of the male will avail until the female has attained a like 

 pitch of preparedness for the work of procreation. The court- 

 ship of the Ruffs and Reeves, already referred to, affords an- 

 other illustration. Here it will be remembered the males 

 for weeks spend laborious days in endeavouring to gain some 

 responsive sign from their prospective but phlegmatic mates, 

 yet without receiving the slightest sign of encouragement or 

 recognition. As soon, however, as the female has become 

 " sexually ripe," as soon as the hormones secreted by her gener- 

 ative glands have done their work, she herself indulges in a 

 species of nuptial dance, waltzing round her lord, and settling 

 down before him with her tail directed towards his head ! 

 Thus the sexual activity displayed by the male comes to mean 

 simply that he is more ardent at this time than his mate. 

 The advantage of this is obvious : for thereby the more vigor- 

 ous males, by proclaiming their desire to pair, defeat their less 

 vigorous rivals, who might otherwise be chosen. The earlier 

 they can take the field, the more persistent their advances, the 

 greater their chances of ultimate success, and this because they 

 slowly instil a preference which cannot be overcome by later 

 and less virile comers. 



Very well. We may now consider the development of the 

 striking secondary sexual characters which the males of so 

 many birds exhibit. These must be regarded as on the same 

 plane as all other structural variations — as fortuitous: and, 

 appearing in ever so slight a degree, they can but help to 

 increase the intensity of the display, and add to the chances 

 of success in such males as they first appeared in. This being 

 so, their further development is inevitable, a development 

 which will only be checked, as we have just remarked, by 

 natural selection. In course of time, if unchecked they be- 



