182 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 
offered 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 
