SEXUAL SELECTION 633 



this rule commonly prevails, so that they tell us nothing 

 about the protective colors being the same in both sexes of 

 desert-birds. Of the other thirteen species, three belong to 

 genera in which the sexes usually differ from each other, 

 yet here they have the sexes alike. In the remaining ten 

 species, the male differs from the female ; but the difference 

 is confined chiefly 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 sur- 

 faces of both sexes have been acted on and rendered alike, 

 through natural selection, for the sake of protection; while 

 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 prevented 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 bril- 

 liant 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. orna- 

 ment. 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 selection, 

 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 song- 

 sters 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 



