BIRDS— COLOR AND NIDIFICATION. 453 



la color between the sexes of the same species are, also, of the 

 same general nature as the differences in color between the dis- 

 tinct species of the same group. For when in a group in which 

 the sexes are usually alike, the male differs considerably from 

 the female, he is not colored in a quite new style. Hence we 

 may infer that within the same group the special colors of both 

 sexes when they are alike, and the colors of the male, when he 

 differs slightly or even considerably from the female, have been in 

 most cases determined by the same general cause; this being 

 sexual selection. 



It is not probable, as has already been remarked, that differ- 

 ences in color between the sexes, when very slight, can be of 

 service to the female as a protection. Assuming, however, that 

 they are of service, they might be thought to be cases of transi- 

 tion; but we have no reason to believe that many species at 

 any one time are undergoing change. Therefore we can hardly 

 admit that the numerous females which differ very slightly in 

 color from their males are now all commencing to become ob- 

 scure for the sake of protection. Even if we consider somewhat 

 more marked sexual differences, is it probable, for instance, that 

 the head of the female chaffinch, — the crimson on the breast of 

 the female bullfinch, — the green of the female greenfinch, —the 

 crest of the female golden-crested wren, have all been rendered 

 less bright by the slow process of selection for the sake of protec- 

 tion? I cannot think so; and still less with the slight differences 

 between the sexes of those birds which build concealed nests. On 

 the other hand, the differences in color between the sexes, whether 

 great or small, may to a large extent be explained on the principle 

 of the successive variations, acquired by the males through sex- 

 ual selection, having been from the first more or less limited in 

 their transmission to the females. That the degree of limitation 

 should differ in different species of the same group will not sur- 

 prise any one who has studied the laws of inheritance, for they 

 are so complex that they appear to us in our ignorance to be 

 capricious in their action.^" 



As far as I can discover there are few large groups of birds in 

 which all the species have both sexes alike and brilliantly col- 

 ored, but I hear from Mr. Sclater, that this appears to be the case 

 with the Musophagse or plantain-eaters. Nor do I believe that 

 any large groups exist in which the sexes of all the species are 

 widely dissimilar in color: Mr. Wallace informs me that the 

 chatterers of S. America (Cotingidse) offer one of the best in- 

 stances; but with some of the species, in which the male has a 

 splendid red breast, the female exhibits some red on her breast; 



^ See remarks to this effect In my work on 'Variation under Domesti- 

 cation,' vol. ii. chap. xii. 

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