450 THE DESCENT OF MAN. 



seek protection by building domed or concealed uests. No one 

 who studies, for Instance, Audubon's account of the differences in 

 the nests of the same species in the Northern and Southern United 

 States,^" will feel any great diflBculty in admitting that birds, 

 either by a change (in the strict sense of the word) of their habits, 

 or through the natural selection of so-called spontaneous varia- 

 tions of instinct, might readily be led to modify their manner of 

 nesting. 



This way of viewing the relation, as far as it holds good, be- 

 tween the bright colors of female birds and their manner of 

 nesting, receives some support from certain cases occurring in 

 the Sahara Desert. Here, as in most other deserts, various birds, 

 and many other animals, have had their colors adapted in a 

 wonderful manner to the tints of the surrounding surface. Never- 

 theless there are, as I am informed by the Rev. Mr. Tristram, some 

 curious exceptions to the rule; thus the male of the Monticola 

 cyanea is conspicuous from his bright blue color, and the female 

 almost equally conspicuous from her mottled brown and white 

 plumage; both sexes of two species of Dromoleea are of a lustrous 

 black; so that these three species are far from receiving protec- 

 tion from their colors, yet they are able to survive, for they have 

 acquired the habit of taking refuge from danger in holes or 

 crevices in the rocks. 



With respect to the above groups in which the females are 

 conspicuously colored and build concealed nests, it is not neces- 

 sary to suppose that each separate species had its nidifying in- 

 stinct specially modified; but only that the early progenitors of 

 each group were gradually led to build domed or concealed nests, 

 and afterwards transmitted this instinct, together with their 

 bright colors, to their modified descendants. As far as it can be 

 trusted, the conclusion is interesting, that sexual selection, to- 

 gether with equal or nearly equal inheritance by both sexes, have 

 indirectly determined the manner of nidiflcation of whole groups 

 of birds. 



According to Mr. Wallace, even in the groups in which the 

 females, from being protected in domed nests during incubation, 

 have not had their bright colors eliminated through natural 

 selection, the males often differ in a slight, and occasionally in a 

 considerable degree, from the females. This is a significant fact, 

 for such differences in color must be accounted for by some of 

 the variations In the males having been from the first limited in 

 transmission to the same sex; as it can hardly be maintained that 

 these differences, especially when very slight, serve as a protec- 



-" See many statements in the 'Ornithological Biography.' See, also, 

 some curious observations on the nests of Italian birds by Bugenio Bet- 

 toni, in the 'Atti della Societa Italiana,' vol. xi. 1869, p. 487. 



