2o6 A HISTORY OF BIRDS 



or some of the Babbling Thrushes, e._i^., Alcippe nipalensis, though 

 numerous instances of such variations could be quoted. The 

 eggs of the Black-cap {Sylvia atricapilla) again, in common 

 with many other species, exhibit three distinct types : in one 

 the ground colour is greyish-white or grey smudged and mottled 

 with brown and lavender ; in another the ground colour is 

 salmon-pink smudged with darker pink and grey, and spotted 

 with reddish-brown ; while in the third type the shell is white, 

 blotched and speckled with chestnut and lavender. Not even 

 a plausible explanation of this variability has yet been found. 

 All three types may occur in the same locality, and they are 

 not correlated in this or any other cases — and they are numer- 

 ous — either with peculiarities of plumage or of the construction 

 of the nest or of its site. 



Yet, generally speaking, the coloration of eggs has a real 

 significance, inasmuch as it is undoubtedly determined to a 

 large extent by the nature of the environment. That is to 

 say, the coloration can be shown to be protective in a very 

 large number of instances. Nowhere is this more certainly 

 true than in the case of such eggs as are deposited on the 

 ground and left for longer or shorter periods completely ex- 

 posed. The eggs of the Plover and Gull tribe constitute the 

 most striking examples of this fact, as those who have tried to 

 find them know well. The eggs of the Marsh-dwellers, since 

 they deposit their treasures on dark, damp ground, have the 

 ground colour of the shell of some dark hue, with darker 

 blotches and spots, while among such as nest on sandy or 

 shingly beaches the ground colour of the shell is correspond- 

 ingly pale, with similar dark markings, and so perfectly do these 

 frail bodies match their surroundings that they are discovered 

 by accident rather than design ; even expert egg-collectors are 

 reduced to adopting various devices to aid them in their search. 



Thus then the curious dissimilarity, the unlikeness which 

 obtains between the coloration of the eggs and of the bird 

 which lays them, becomes no longer a matter for wonderment ; 

 in both instances they are the outcome of the needs of the 

 environment, of mechanical selection. Both eggs and birds 

 have enemies, and the device — if " device " it may be called — 

 of protective coloration is one of several ways for circumvent- 

 ing these enemies. As the size and shape and relation to the 



