72 EVOLUTION AND ANIMAL LIFE 



those specially connected with the care and rearing of the 

 young; as the mammae of female mammals, the brood pouches 

 of the female kangaroos and opossums, etc. But a moment's 

 reflection calls to mind the existence of a host of other differ- 

 ences between males and females of the same species which 

 plainly have no such immediate relation to the distinct functions 

 or duties assumed by each in the business of production and 

 care of young. For example, the long plume feathers of the 

 male bird of paradise, the curious chitinous horns of the male 

 leaf-chafer beetles (Fig. 41), the brilliant plumage of many male 

 birds as contrasted with the sober dress of the females, and a 

 host of other distinguishing characteristics of the sexes in many 

 animal species. Now these differences are all conveniently 

 named by the phrase " secondary sexual differences," and the 

 explanation of their origin has come to be one of the most 



FIG. 41. Male and female Scarabeid beetles, Phnneus mexicanus, showing sex dimor- 

 phism; the male with prominent dorsal horn on head. (From specimens.) 



puzzling of biological problems. The most familiar and, for 

 many years, a widely accepted solution of this problem, is that 

 embraced in the theory of sexual selection proposed and fought 

 for by Darwin and Wallace, but later discarded by the latter 

 of these great naturalists. 



Before taking up the sexual selection explanation of dis- 

 tinguishing sex characters, it is well to pay a little further 

 attention to the characters themselves. And for this purpose a 

 rough grouping or classification may be attempted. 



The characters may be of special use to the possessor (male 

 or female) or for the benefit of the young, such as weapons of 

 offense and defense (antlers of male deer, stings of female bee 

 and wasp, tusks of male swine, etc.), or special organs for mat- 

 ing (seizing and holding organs of certain male crabs, suckerlike 

 holding pads on the feet of male water beetles (Fig. 42), or special 

 locomotory organs (presence of wings in the male and their 



