B. -Hamilton — Winter Whitening of Maminah and Birch. 307 



heat aud cold exert very serious influences on acimal organisms. In 

 the human body, for instance, " continued exposure to such degrees of 

 cold as is yet not incompatible with the maintenance of life, neverthe- 

 less keeps at low ebb activity of nutrition and function alike " 

 (Dr. A. E; Durham in Quain's Medical Dictionary, p. 270). The action 

 of the skin is sluggish, that of the kidneys more active. Under an 

 increase of temperature, on the contrary, the exhalation of carbonic 

 acid and of water is lessened ; the urine diminishes in quantity and 

 contains less urea and chlorides. But the skin acts much more freely, 

 its secretion being increased by about 24 per cent. 



One of the concomitants of a sluggish metabolism is diminished 

 oxydisation and consequent storage of fat. For instance, fat is readily 

 accumulated by castrated animals in contrast to those which are in 

 full sexual activity. Many animals accumulate fat during one season 

 of the year, and utilize it during the breeding season. In particular, 

 this has been shown in considerable detail by Dr. Noel Paton and his 

 colleagues of the Fishery Board for Scotland to be true of the Salmon, 

 while the African Mud-Fish {Protopterus) is another well-known 

 example. 



I have already shown the connexion between fat and animal 

 pigmentation. It seems then not an unreasonable suggestion that the 

 temporary cessation of metabolism of fat and the absence of pigment 

 are part of the same process. In the autumnal season the metal)Glism 

 grows more and more sluggish, particularly in the periphery of the 

 body, until there comes a time when a maximum of fat remains idle 

 internally. 



There is, as it were, to use a graphic expression, at one time a 

 centrifugal, at another a centripetal, condition of fat. And, since the 

 pigment accompanies the fat, any hairs or feathers which grow 

 during the prevalence of the centripetal condition are white to an 

 extent which is evident more or less according to the intensity of 

 the physiological influences at work. Hence results a condition of 

 things wherein exists great opportunity for the play of such diverse 

 factors as heredity, individual temperament, and the influence of 

 external conditions. This exactly corresponds to the observed facts, 

 always so puzzling, often at first sight so contradictory. 



But not only are new hairs white. Eventually there comes a time 

 when, with a constantly lowering metabolism, not only is peripheral 

 activity sluggish, but, as shown by Mr. Barcroft, material is actually 

 recalled from the hairs, no doubt for the internal uses of the body. 

 The working of this process has been observed by Professor 



