216 
DARWINISM 
CHAP. 
become practically invisible among the complex lights and 
shadows of the foliage they feed upon. 
In the case of the cuckoo, which lays its eggs in the nests 
of a variety of other birds, the eggs themselves are subject 
to considerable variations of colour, the most common type, 
however, resembling those of the pipits, wagtails, or warblers, 
in whose nests they are most frequently laid. It also often 
lays in the nest of the hedge-sparrow, whose bright blue eggs 
are usually not at all nearly matched, although they are 
sometimes said to be so on the Continent. It is the opinion 
of many ornithologists that each female cuckoo lays the same 
coloured eggs, and that it usually chooses a nest the owners 
of which lay somewhat similar eggs, though this is by no 
means universally the case. Although birds which have 
cuckoos’ eggs imposed upon them do not seem to neglect them 
on account of any difference of colour, yet they probably do 
so occasionally ; and if, as seems probable, each bird’s eggs are 
to some extent protected by their harmony of colour with their 
surroundings, the presence of a larger and very differently 
coloured egg in the nest might be dangerous, and lead to the 
destruction of the whole set. Those cuckoos, therefore, which 
most frequently placed their eggs among the kinds which they 
resembled, would in the long run leave most progeny, and 
thus the very frequent accord in colour might have been 
brought about. 
Some writers have suggested that the varied colours of 
birds’ eggs are primarily due to the effect of surrounding 
coloured objects on the female bird during the period pre¬ 
ceding incubation; and have expended much ingenuity in 
suggesting the objects that may have caused the eggs of one 
bird to be blue, another brown, and another pink. 1 But no 
evidence has been presented to prove that any effects what¬ 
ever are produced by this cause, while there seems no difficulty 
in accounting for the facts by individual variability and the 
action of natural selection. The changes that occur in the 
conditions of existence of birds must sometimes render the 
concealment less perfect than it may once have been ; and 
when any danger arises from this cause, it may be met either 
1 See A. H. S. Lucas, in Proceedings of Royal Society of Victoria, 1887, 
p. 56. 
