324 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



Natural Selection does not bring about absolutely 

 perfect adaptation, but only so far moulds a species 

 to the conditions of life as to enable it to live and 

 thrive. White eggs without protective colouring may 

 be only a slight source of danger and, so, continue, 

 just as we often find an organ continuing in a useless 

 rudimentary state, not eliminated in the struggle for 

 existence, because, if useless, it is also almost harmless. 

 It must not be supposed that the production of colour 

 puts a great strain upon the system. It is merely a 

 waste product of the body, which otherwise would be 

 made no use of. The pigments are very similar to 

 those of the bile, though they are not exactly identical. 



Natural Selection has no doubt in many cases 

 turned colours to account. It is impossible to deny 

 that they are in some cases protective. But imagine 

 the case of a species which at one time needed pro- 

 tective colouring for its eggs, and which afterwards 

 changed its nesting habits, or was relieved of the 

 presence of some enemy that preyed upon its eggs, 

 so that the protection became unnecessary ; then it is 

 highly probable that the old tint and markings would 

 continue because of their harmlessness, though they 

 had ceased to be beneficial. This may be the reason 

 of the protective coloration of the eggs of Gulls ; it 

 explains, too, the want of it in the case of the eggs 

 of the Short-eared Owl. The old style of egg common 

 to the family has been maintained because, though in 

 this species some colour might be a slight advantage, 

 the question has not become a burning one. 



If Natural Selection is so slow to act upon them, 

 how is it that the colours of eggs arc as constant as 



