BIRDS' EGGS 277 



arise long streaks, hair lines, and smeared splashes. That 

 the pattern is often very pleasing every one admits. 



The pigments are related to those of the blood and the 

 bile, e.g. oorhodein and biliverdin respectively ; but their 

 primary meaning remains obscure. It is very likely that 

 they are unimportant by-products or waste-products of the 

 bird's metabolism, which are got rid of along with the all- 

 important nutritive secretions from the wall of the oviduct. 

 The pigments in withering leaves are very beautiful and very 

 striking, but, so far as we know, they are devoid of biological 

 significance except as end-products and by-products in the 

 essential chemical routine of the green leaf. 



Three points seem to us of much importance. The first 

 is that of variability. The beautiful collections generously 

 given by Mr. R. Hay Fenton to the Aberdeen University 

 Museum show the extraordinary variety of coloration and 

 pattern in the eggs of guillemot, lapwing, black-headed gull, 

 rook, and cuckoo. To look at a drawer of a score of different 

 kinds is Hke sitting by a fountain of change, and there are 

 many such drawers. These cases show that it need not be 

 of much importance what colours and pattern the egg has. 

 Within wide limits the secretion in the oviduct may vary, 

 and it is known experimentally that feeding or fright, age 

 or other nurtural conditions may affect the coloration of a 

 bird's egg. If it should become of survival-value that the 

 egg of a guillemot or a cuckoo, let us say, should settle down 

 to some particular coloration, there is plenty of raw material 

 on which the process of natural selection could work. 



The second point is specificity — that on the whole there 

 is distinctive coloration for each species. Those who 

 make a study of birds' eggs do not usually make a mistake, 

 unless they come across some very divergent variation 

 or freak. Each species is itself and no other. Is it not the 

 case that the shell-membrane in the guillemot's egg is 

 almost always yellow, while that of the nearly related 

 razorbill, often alike in external coloration of shell, is 

 green ? The specificity may simply mean that a certain type 

 of constitution has been stabilised — similar metabolism, 



