Page:Descent of Man 1875.djvu/474

458 there is a similar difference, the face and wing-coverts moreover of the female being of a paler blue than in the male. In the family of the tits (Parinæ), which build concealed nests, the female of our common blue tomtit (Parus cæruleus) is "much less brightly coloured" than the male; and in the magnificent Sultan yellow tit of India the difference is greater.

Again in the great group of the woodpeckers, the sexes are generally nearly alike, but in the Megapicus validas all those parts of the head, neck, and breast, which are crimson in the male are pale brown in the female. As in several woodpeckers the head of the male is bright crimson, whilst that of the female is plain, it occurred to me that this colour might possibly make the female dangerously conspicuous, whenever she put her head out of the hole containing her nest, and consequently that this colour, in accordance with Mr. Wallace's belief, had been eliminated. This view is strengthened by what Malherbe states with respect to Indopicus carlotta; namely, that the young females, like the young males, have some crimson about their heads, but that this colour disappears in the adult female, whilst it is intensified in the adult male. Nevertheless the following considerations render this view extremely doubtful: the male takes a fair share in incubation, and would be thus almost equally exposed to danger; both sexes of many species have their heads of an equally bright crimson; in other species the difference between the sexes in the amount of scarlet is so slight that it can hardly make any appreciable difference in the danger incurred; and lastly, the colouring of the head in the two sexes often differs slightly in other ways.

The cases, as yet given, of slight and graduated differences in colour between the males and females in the groups, in which as a general rule the sexes resemble each other, all relate to species which build domed or concealed nests. But similar gradations may likewise be observed in groups in which the sexes as a general rule resemble each other, but which build open nests. As I have before instanced the Australian parrots, so I may here-instance, without giving any details, the Australian pigeons. It deserves especial notice that in all these cases the slight