486 THE DESCENT OP MAN. 



remaining ten species, the male differs from the female; but the 

 difference is confined ohiefly to the under surface of the plumage, 

 which is concealed when the bird crouches on the ground; the 

 head and back being of the same sand-colored hue in the two 

 sexes. So that in these ten species the upper surfaces of both 

 sexes have been acted on and rendered alike, through natural 

 selection, for the sake of protection; whilst the lower surfaces of 

 the males alone have been diversified, through sexual selection, 

 for the sake of ornament. Here, as both sexes are equally well 

 protected, we clearly see that the females have not been pre- 

 vented by natural selection from inheriting the colors of their 

 male parents; so that we must look to the law of sexually-limited 

 transmission. 



In all parts of the world both sexes of many soft-billed birds, 

 especially those which frequent reeds or sedges, are obscurely 

 colored. No doubt if their colors had been brilliant, they 

 would have been much more conspicuous to their enemies; but 

 whether their dull tints have been specially gained for the sake 

 of protection seems, as far as I can judge, rather doubtful. It 

 is still more doubtful whether such dull tints can have been 

 gained for the sake of ornament. We must, however, bear in mind 

 that male birds, though dull-colored, often differ much from 

 their females (as with the common sparrow), and this leads to 

 the belief that such colors have been gained through sexual selec- 

 tion, from being attractive. Many of the soft-billed birds are 

 songsters; and a discussion in a former chapter should not be 

 forgotten, in which it was shown that the best songsters are 

 rarely ornamented with bright tints. It would appear that female 

 birds, as a general rule, have selected their mates either for their 

 sweet voices or gay colors, but not for both charms combined. 

 Some species, which are manifestly colored for the sake of pro- 

 tection, such as the jack-snipe, woodcock, and night-jar, are like- 

 wise marked and shaded, according to our standard of taste, with 

 extreme elegance. In such cases we may conclude that both 

 natural and sexual selection have acted conjointly for protection 

 and ornament. Whether any bird exists which does not possess 

 some special attraction, by which to charm the opposite sex, 

 may be doubted. When both sexes are so obscurely colored that 

 it would be rash to assume the agency of sexual selection, and 

 when no direct evidence can be advanced showing that such 

 colors serve as a protection, it is best to own complete ignorance 

 of the cause, or, which comes to nearly the same thing, to at- 

 tribute the result to the direct action of the conditions of life. 



Both sexes of many birds are conspicuously, though not bril- 

 liantly colored, such as the numerous black, white, or piebald 

 species; and these colors are probably the result of sexual selec- 

 tion. With the common blackbird, capercailzie, blackcock, black 



