176 THE BIOLOGY OF THE SEASONS 



splashes, or long streaks, or irregular blotches often 

 conspicuous against a ground colour of white or of some 

 previously deposited pigment. This is only the A B C of 

 a difficult matter ; but with this simple fact clearly focussed 

 in the mind, it is very instructive to look over a collection 

 of eggs and interpret the precise well-fixed marks, the 

 clear-cut edges, the hair-lines, the spirals, the long-drawn- 

 out splashes, as indicative of the rest or the movement of 

 the egg during the period of painting. In many cases it 

 has obviously moved much before the paint was dry. In 

 this connection we must remember the general uniformity 

 of natural process. Given the same kind of bird, the same 

 size of egg, the same calibre of oviduct, the same pigments, 

 the same rate of egg-movement, and the coloration of 

 the egg will tend to be the same. And it is interesting to 

 know experimentally that abnormal conditions, such as 

 fright or captivity, may disturb the natural painting of 

 the egg-shell, or suppress it altogether. Thus we have seen, 

 for instance, a woodcock's clutch with pure white eggs- 

 most unprofitably conspicuous in the wood. 



The next point of importance is that the coloration of 

 birds' eggs is variable, and sometimes very markedly so. 

 Notoriously variable are the eggs of the Black-Headed Gull 

 (Larus ridibundus) from unspotted pale blue at one 

 extreme to very dark brown, with darker spots, at the 

 other. The eggs of the rook, the guillemot, and the cuckoo 

 are also good illustrations of variability. The bird cannot 

 alter the colour of the egg by willing it ; she is not influenced 

 by surrounding colours ; the eggs are only to a slight extent 

 modifiable by external influences. There is no doubt that 

 the variability in colouring is due to a peculiarity in the 

 constitution of the mother-bird. The important fact has 

 been noticed that, when a mother-bird has begun to lay eggs 

 of a peculiar coloration, she may continue to do so for 



