1 82 THE DOCTRINE OF DESCENT. 



They need not bury themselves entirely, for it is only 

 by close examination that their bare skin can be distin- 

 guished from the sandy bottom, and under this partly 

 artificial, partly natural veil and mask, the animal waits 

 for its prey. 



In many animals provided with protective colouring 

 the phenomena are more complex, and explanation by 

 natural selection is far more difficult; for they are able 

 voluntarily to adapt their colour to circumstances, or else 

 their colour changes by involuntary reflexes. Verany's 

 unsurpassable observations on the Cephalopoda have ac- 

 quainted us with the range of colours at the disposal of 

 these Molluscs, and to this may be joined Brehm's 

 description of the changes of colour in the chameleon. 

 On these highly complex cases some light is thrown by 

 the simpler instances in which the manifestly protective 

 colouring has become fixed in skin and plumage, and 

 the concurrence of other circumstances scarcely admits 

 of any other explanation than selection. 



On this point, Wallace's interesting researches on 

 bird's-nests are especially instructive. The great ma- 

 jority of female birds which sit in open nests possess 

 brown or gray, in short, unobtrusive plumage. No con- 

 tradiction will be offered to the statement that any casual 

 modifications of plumage, which would more readily be- 

 tray the sitting bird to its enemies, would have no pros- 

 pect of becoming constant. The converse follows 

 naturally with regard to colouring which brings the bird 

 into harmony with its environment; and an important 

 guarantee of the correctness of this interpretation of facts 

 is afforded by the other observation, that most female 

 birds with gaily coloured or speckled plumage sit in cov- 



