234 SEXUAL selection: birds. PartIL 



exhibit a marked preference or antipathy for certain 

 individual males. 



If it be admitted that the females prefer, or are 

 unconsciously excited by the more beautiful males, then 

 the males would slowly 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 in which the 

 sexes differ, the males differ much more from each other 

 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 dis- 

 tinct. Birds in a state of nature offer individual differ- 

 ences which would amply suffice for the work of sexual 

 selection ; but we have seen that they occasionally pre- 

 sent more strongly-marked variations which recur so 

 frequently that they would immediately be fixed, if 

 they served to allure the female. The lavvs of variation 

 will have determined the nature of the initial changes, 

 and largely influenced the final result. The grada- 

 tions, which may be observed between the males of 

 allied species, indicate the nature of the steps which 

 have been passed through, and explain in the most 

 interesting manner certain characters, such as the 

 indented ocelli of the tail-feathers of the peacock, and 

 the wonderfully-shaded ocelli of 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 



