CHAPTER V. 



DARWINISM ATTACKED (CONTINUED) : THE 

 THEORY OF SEXUAL SELECTION. 



THE differences between male and female individuals of 



a single species are often striking; recall the gorgeous 



colouring, the plumes and tufts and tail-feathers 



Secondary 



sexual differ- of many male birds compared with the sober 



ences. 



and quiet plumage of their mates ; the antlers 

 of the stag, the mane of the lion and bison, the beard of 

 the goat, many monkeys, and of man. Recall the mammae 

 of the female quadrupeds, the brood pouches of the female 

 kangaroos and opossums, the small size, compared with their 

 mates, of many female birds, the winglessness of many fe- 

 male insects. Other less familiar kinds of animals show 

 sexual dimorphism or dichromatism in even more striking 

 degree, while in many others the differences are less con- 

 spicuous but nevertheless perfectly obvious if some attention 

 is given to looking for them. These differences in size, 

 colour, general appearances, and various specific structural 

 details in head, trunk, wings, feet, plumage, etc., are over 

 and beyond those primary radical differences existing in all 

 species in which the two sexes are differentiated. Some of 

 these differences may, however, have obvious relation to 

 the primary differences, in that they may be connected im- 

 mediately with the act of pairing or with the work of rear- 

 ing the young. The presence in male insects of complexly 

 developed holding organs, and in female mammals of milk 

 glands exemplifies differences of this category. A great many 

 sexual differences, however, have no such obvious direct 

 relation to the function of producing and rearing the young. 



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