BIRDS' EGGS 177 



years. The pleasant brown colour of the eggs of some 

 fowls is due to a variation which breeds true and does 

 not blend ; by taking certain well-known precautions in 

 breeding it can be, as it were, grafted on to a stock laying 

 white eggs. 



We have described in a simple way the mechanism by 

 which birds' eggs are coloured ; we have seen that most 

 of the pigments are allied to the red pigment of the 

 blood ; we have noticed that in many cases birds have not 

 yet quite settled down to a stereotyped kind of egg-colora- 

 tion ; we must now ask whether the colouring has any 

 selective value, or, in other words, any vital significance. 



In some cases, where the eggs are well hidden or are 

 almost quite safe, it is difficult to find any significance in 

 the colouring. It may express the unity of the organism, 

 but it seems to be useless in the present condition of the 

 species. Similarly, the colouring of deep-sea animals, or 

 the colouring of withered leaves, cannot be shown to have 

 any direct utility. Similarly, some of the internal organs 

 of animals have very fine colours, absolutely determined, 

 never haphazard, but yet valueless as regards optical 

 effect, because invisible. 



On the other hand, it seems quite impossible to doubt 

 the occasionally great vital value of the coloration which 

 in other cases may be a negligible feature. We refer, 

 of course, to those cases where exposed eggs are incon- 

 spicuous because their coloration harmonises so well with 

 the surroundings. This is particularly striking in many 

 members of the Plover and Gull tribes, where it is almost 

 impossible to exaggerate the difficulty of finding the eggs 

 in their appropriate surroundings. This is a particularly 

 instructive case to think over, for one can understand that, 

 given an abundant crop of colour- variations and a severe 

 elimination-process, survival would be with the appropri- 



