84 A HISTORY OF BIRDS 



ings. To this we shall return in a later chapter. But here 

 we would remark that the evidence seems rather to show that 

 the action of natural selection followed another line — the 

 physiological reaction of the organism to heat and a dry atmo- 

 sphere, which seem, in some way, inimical to the production 

 of pigment, except in a dilute form such as would give rise to pale 

 coloration. How much is' due to the direct action of the sun's 

 rays on the ceratin which forms so large a portion of the 

 superficial part of the feather remains to be discovered. But 

 it would seem that this is a factor to be seriously considered. 

 That there is a direct relation between high temperature and a 

 dry atmosphere on the one hand, and weak pigmentation on the 

 other, seems certain. 



To suppose that the phenomenon of bleaching as here set 

 forth is a concession to Lamarckism would be to fall into a 

 grievous error. The reduction of colour is due either to the 

 selection of individuals with reduced powers of pigment secre- 

 tion which may be correlated with other qualities necessary for- 

 such an environment, and therefore be of blastogenetic origin ; 

 or it may be somatogenic, that is to say, a purely physiological 

 phenomenon, impressed anew upon each generation ; in other 

 words this coloration is not transmissible. 



Let us pass now to a review of the evidence as to the 

 opposite relation between extreme cold and pigmentation. 



As is well known, the birds of Alpine and Arctic Regions 

 are commonly white, at least during the winter months, while 

 during the rest of the year they may wear a plumage quite 

 richly coloured. The Ptarmigan and Willow-grouse are cases 

 in point. These birds, at the approach of winter, gradually 

 discard their richly mottled plumage and assume a dress of 

 pure white, though it has been stated that this white dress is 

 assumed, not by a moult, but by the ingestion of the pigment 

 of the feathers by phagocytes. This, however, does not appear 

 to be supported by fact, for in the first place the moulting has 

 been watched repeatedly, and in the second it seems improbable 

 that phagocytes would be able to make their way up the tissue 

 of a feather. That this whiteness is the direct result of a nicely 

 sensitive reaction to cold is shown by the fact that the change 

 varies in completeness according to the intensity of the stimulus. 

 But since it does not manifest itself in all Alpine and Arctic 



