NATURAL SELECTION; SEXUAL SELECTION 



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

 problem. Although indeed the reason why these useful cliar- 

 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 cliar- 

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

 ently disadvantageous differences. Wliy the male ])ird 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 wciglit 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 tlie l)ir(l 

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



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Fig. 45. — Male and female argus pheasant; the male is showTi in characteristic "courting 

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



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

 traordinary secondary sexual difforonces the tlicory of sexual 

 selection has been devised. 



This theory, in few worch^ is that there is ])ractically a 

 competition or struggle for mating, and tliat those males are 



