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 are as constant as 
