REPRODUCTION— CONCERNING EGGS 209 



say, that birds lay white eggs because they lay them in holes, 

 inasmuch as in such dimly lighted places coloured eggs, from 

 their lower refracti\e power, would run grave risks of being 

 broken whenever the bird entered the nest, while white eggs, in 

 this dim religious light, are just visible. For a somewhat similar 

 reason plants that are fertilised by night-feeding insects have 

 white flowers, white being conspicuous in the dark, whilst colour 

 is invisible. We ourselves adopt a similar expedient for making 

 objects conspicuous in dark places. Thus in the dark corridors 

 of the British Museum of Natural History a piece of white paper 

 is pasted around the keyholes of doors to serve as a guide there- 

 to, while on underground railways the edges of the platforms and 

 the stairs leading thereto are similarly painted white. By way of 

 supporting this hypothesis as to the significance of white eggs, we 

 may cite the case of the British Puffin whose eggs are white, and 

 deposited in burrows. But if these eggs be examined they will be 

 found to be but thinly covered with a layer of white over a 

 coloured surface, recalling that of the eggs of the Guillemot and 

 Razor-bill. Here then it would seem the custom of laying in 

 burrows is of recent origin, and that selection has begun the work 

 of suppressing the colour, and we may therefore assume, with 

 some justification, that the white eggs of other hole-breeders 

 have similarly acquired a secondary colourlessness, or in other 

 words, have re-acquired the primitive white colour: or it may 

 be that this has been brought about not as in the Puffin but by 

 reversion, though in the majority of cases, as vi^e have already 

 suggested, this development of colour may never have taken 

 place since the race began. 



But some birds, it may be objected, whose eggs are remark- 

 able for the richness and beauty of their coloration, yet lay them in 

 holes. The Snow-bunting {PlectropJianes nivalis) would appear 

 to be a case in point. When, however, we come to examine 

 this, we find that though such nesting sites are used in Scotland, 

 in the tundra of Siberia they deposit their eggs on the ground, 

 concealed amid tussocks of grass. Thus the breeding habit of 

 Scotch birds is exceptional, and determined by the nature of 

 the environment. It may well happen that in course of time 

 those which persist in breeding in holes will come to lay white 

 eggs like the Puffin. Another objection which may be raised 

 is that many birds, as the Pigeons, for example, lay white eggs 

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