62 THE COLOUES OF ANIMALS 



of great interest; protective colouring can never be 

 fully understood until this principle is taken into 

 account. 



In order to make out the true meaning of the 

 colours of eggs they must be observed in their natural 

 surroundings, and must be looked at from all points 

 of view and at varying distances. It is very probable 

 that the bright blue colour of certain eggs will be 

 explicable under these conditions. 



The fact that concealed eggs are almost invariably 

 white strongly confirms the conclusion that the colours 

 of exposed eggs are of value to the species, and are 

 maintained by the operation of natural selection. 

 Certain exposed eggs may, however, be white, as in 

 the wood-pigeon, but in these cases the eggs are pro- 

 tected from enemies beneath ; for the holes in the 

 loosely constructed nest through which they are seen 

 cannot be distinguished from others through which 

 the bright sky appears. 



The whiteness of eggs hidden in holes or in covered 

 nests, or buried under leaves, is of a very different 

 nature, for it is due to the cessation of natural selec- 

 tion, perhaps aided by reversion to the ancestral 

 colour, which is still preserved in the eggs of reptiles. 

 All useful characters are kept up to a high pitch of 

 efficiency by the continual elimination of the unfittest, 

 and as soon as such elimination ceases, the level of 

 efficiency must fall. This interpretation is confirmed 

 by the fact that the eggs of certain species which now 



