216 DARWINISM 



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 A\Titers 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.^ 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 



^ See A. H. S. Lxicas, in Proceedings of Royal Society of Victoria, 1887, 

 p. 56. 



