224 SEXUAL SELECTION: BIRDS. [Part IL 



state of nature offer individual differences which would 

 am])ly suffice for the work of sexual selection ; but we 

 have seen that they occasionally present more strongly- 

 marked variations, which recur so frequeutly that they 

 would immediately be fixed, if they served to allure the 

 fenuile. The laws of variation will have determined the 

 nature of the initial changes, and largely influenced the 

 final result. The gradations, which may be observed be- 

 tween 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 colors, 

 top-knots, fine plumes, etc., of many male birds cannot 

 have been acquired as a protection ; indeed, they some- 

 times 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 pro- 

 duced some definite effect on both sexes, the more impor- 

 tant result will have been an increased tendency to fluc- 

 tuating variability or to augmented individual dift'erences ; 

 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 chai-acters ac- 

 quired by the males for the sake of ornament, for pro- 

 ducing various sounds, and for fighting 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 sometimes have 

 been transmitted in one way and sometimes in another is, 



