Chap. XVI. Birds — Summary. 497 



excited by the more beautiful males, then the males would 

 Blowly but surely be rendered more and more attractive through 

 sexual selection. That it is this sex which has been chiefly 

 modified, we may infer from the fact that, in almost every genus 

 where the sexes differ, the males differ much more from one 

 another than do the females ; this is well shewn in certain 

 closely-allied representative species, in which the females can 

 hardly be distinguished, whilst the males are quite distinct. 

 Birds in a state of nature offer individual differences which 

 would amply suffice for the work of sexual selection; but we 

 have seen that they occasionally present more strongly-marked 

 variations which recur so frequently that they would imme- 

 diately be fixed, if they served to allure the female. The laws 

 of variation must determine the nature of the initial changes, 

 and will have largely influenced the final result. The grada- 

 tions, which may be observed between the males of allied 

 species, indicate the nature of the steps through which they 

 have passed. They explain also in the most interesting manner 

 ^ow certain characters have originated, such as the indented 

 acelli on the tail-feathers of the peacock, and the ball and 

 socket ocelli on the wing-feathers of the Argus pheasant. It is 

 evident that the brilliant colours, top-knots, fine plumes, &c., of 

 many male birds cannot have been acquired as a protection; 

 indeed, they sometimes lead to danger. That they are not due 

 to the direct and definite action of the conditions of life, we may 

 feel assured, because the females have been exposed to the same 

 conditions, and yet often differ from the males to an extreme 

 degree. Although it is probable that changed conditions acting 

 during a lengthened period have in some cases produced a 

 definite effect on both sexes, or sometimes on one sex alone, the 

 more important result will have been an increased tendency to 

 vary or to present more strongly marked individual differences ; 

 and such differences will have afforded an excellent ground- 

 work for the action of sexual selection. 



The laws of inheritance, irrespectively of selection, appear to 

 have determined whether the characters acquired by the males 

 for the sake of ornament, for producing various sounds, and for 

 lighting together, have been transmitted to the males alone or to 

 both sexes, either permanently, or periodically during certain 

 seasons of the year. "Why various characters should have been 

 transmitted sometimes in one way and sometimes in another, is 

 Qot in most cases known ; but the period of variability seems often 

 to have been the determining cause. When the two sexes have 

 inherited all characters in common they necessarily resembk 

 each other ; but as the successive valuations may be differently 

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