B. -Hamilton — Winter Whiteniny of Mammals and Birds. 305 



nitrogen. Almost all other vertebrate colours are structural. The 

 basis of these structural colours is, however, always a strong deposit 

 of pigment. 



Finally, we have evidence of the direct part played by fat in 

 animal colouration in the marked change which may be brought about 

 in the plumage of birds, such as the Bullfinch, by the ingestion of a 

 fatty food such as hempseed. 



It has been pointed out to me^ that the pigmentation of the skin 

 which accompanies the peripheral distribution of fat in human 

 pregnancy is a fact which supports my views. Again, in the morbid 

 symptoms of Addison's disease abnormal pigmentation is remarkably 

 associated with abundance of fat, especially distributed subcutaneously 

 on the abdomen. Most striking of all, perhaps, is the removal of fat, 

 and with it pigment, from certain organs of the breeding Salmon 

 (muscles, intestines, and liver) for deposition in the ovaries and the 

 skin. The result is that these organs become pigmented, to use Miss 

 Newbiggin's expression, " as it were incidentally in the life-history 

 of the individual under circumstances which render the question as 

 to the inheritance of acquired characters absolutely unimportant. "- 



It seems, then, hardly possible to avoid connecting this pigmented 

 fat of animals with their external coloiu-ation. The suggestion is not, 

 I think, a new one. 



II. The connexion between Fat and Winter Whitening. 



Let us now apply these facts to the Arctic mammalia and birds, 

 iirst briefly recapitulating the known phenomena of winter whitening. 

 The phenomena are not nearly so isolated as seems to be generally 

 believed. From the always pure white (or rather yellowish) Polar 

 Bear and the Snowy Owl, through the seasonably white Polar and 

 Alpine Hares and Ptarmigans, there are many intermediate degrees of 

 winter whitening, until the commencement of the process is just 

 visible in the numerous instances where northern animals wear a 

 winter coat or plumage lighter than that of summer (Squirrels, 

 Auks, Guillemots, &c.). 



The manner of this change of colour has been deeply discussed, 

 and has caused much disagreement. The Polar Hare, in particular, 



ifiy Dr. Gadow and Mr. Anderson, of Cambridge, who have been so kind 

 -as to read my paper. 



2 See Report of Fishery Board for Scotland for 1S9S. 



2 02 



