1 82 THE DOCTRINE OF DESCENT. 



frequently kept in aquaria. Observe the gray or 

 brownish speckled creatures, as with a few strokes of 

 their fins they partially cover their upper surface with 

 sand. They need not bury themselves entirely, for it is 

 only by close examination that their bare skin can be 

 distinguished 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 Cephalo- 

 poda have acquainted us with the range of colours 

 at the disposal of these Molluscs, and to this may be 

 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 circum- 

 stances scarcely admits of any other explanation than 

 selection. 



On this point, Wallace's interesting researches on bird's- 

 nests are especially instructive. The great majority of 

 female birds which sit in open nests possess brown or gray, 

 in short, unobtrusive plumage. No contradiction will be 

 off"ered to the statement that any casual modifications 

 of plumage, which would more readily betray the sitting 

 bird to its enemies, would have no prospect of becoming 

 constant. The converse follows naturally with regard 

 to colouring which brings the bird into harmony with 



