110 SEXUAL DIMORPHISM 



the males consist more in characters whose function is to act 

 upon the senses of the female, than those used in fighting. 

 There are reasons for this in the main structural character- 

 istics of the two classes. The viviparous mode of reproduc- 

 tion in mammals has led to a periodical congestion of the 

 female generative organs, and to physiological processes in 

 them which act powerfully on the nervous system, so that 

 copulation in most cases takes place only when the female 

 organs are in a particular state, and at that season the female 

 has almost as much desire as the male. The mammal also has 

 from his structure more power of grasping the female than 

 the male bird. In birds the consent of the female is both 

 more necessary and more difficult to obtain, and therefore 

 the male has acquired the habit of appealing to the eyes of 

 the female by gestures of courtship, etc. The degrees in the 

 development of special organs of courtship, however, will be 

 found to correspond to the proportion of his time and energies 

 which the male habitually devotes to courtship. This will 

 be found to be a principle of general application, holding 

 good both for polygamous and monogamous species, while, as 

 Darwin admitted, sexual dimorphism is not exclusively asso- 

 ciated with polygamy. The actions connected with nest- 

 building and incubation are sometimes performed exclusively 

 by the female, as in Phasianidse ; sometimes by both sexes 

 equally, as in pigeons ; sometimes by the male alone, as in 

 Turniv ; and the degree to which the male or female is special- 

 ised for courtship exactly corresponds to these differences. 



To some minds, however, it may appear difficult to under- 

 stand why a male bird should commence the habit of erecting 

 and displaying particular feathers in the beginning, when he 

 had no special plumage to display, and when, therefore, the 

 erecting or spreading of his feathers would have no effect 

 upon the feelings of the hen. This difficulty arises entirely 

 from the hitherto accepted ideas concerning the process of 



