NATURAL SELECTION; SEXUAL SELECTION 



75 



means of locomotion, etc., may be considered to offer no special 

 problem. Although indeed the reason why these useful char- 

 acteristics should be possessed by but one sex is by no means 

 always, or perhaps even often, plain to us. 



But the real problem presented by secondary sexual char- 

 acters is that thrust on us by the nonuseful and even appar- 

 ently disadvantageous differences. Why the male bird of para- 

 dise should be decked out in a plumage certain to make it 

 a conspicuous object to every enemy it has, and of a weight and 

 difficulty of manipulation that must mean a constant demand 

 on the strength and attention of the bird, is a question that 

 demands a special answer. In the same case with the bird 

 of paradise are the peacock, the gorgeous male pheasant (Fig. 45) , 



FIG. 45. Male and female argus pheasant; the male is shown in characteristic "courting 

 attitude." (From Tegetmeier's " Pheasants.") 



many hummingbirds (Fig. 40), etc. Now to explain these ex- 

 traordinary secondary sexual differences the theory of sexual 

 selection has been devised. 



This theory, in few words, is that there is practically a 

 competition or struggle for mating, and that those males are 



