ON INCUBATION. 145 



of the eggs, but I did not ascertain precisely what it was. 

 If there is always a considerable interval (and this does 

 not seem unlikely when the size of the egg compared to 

 the bulk of the bird is taken into consideration), it is 

 evident that eggs left exposed in this manner run great 

 risks of being destroyed, and it is certain that any variation 

 which tended to make them at all conspicuous would 

 result in the whole clutch disappearing. 



The range of variation in the colouring of the eggs 

 of such a species as the Black-headed Gull is very striking 

 when compared with the uniformity in the colouring of 

 the eggs of the Curlew. 



