306 Proceedings of the Royal Irkh Academy. 



has formed the subject of many contradictory assertions. "WTiatever may 

 be true of other species, Mr. J. Barcroft could find no trace of an autumn 

 moult in the case of a Scotch Yariable Hare, kept at my request at 

 Cambridge under constant observation in the Physiological Laboratory 

 during the autumn of 1899, and which had turned almost completely 

 white by Januaiy of 1900. In tbe very same winter a wild Yariable 

 Hare which lived close to this house turned from half to almost 

 completely white within the space of a few days in late December. 

 The first half of the change had been accomplished earlier in the 

 season. It is, therefore, very hard to believe that the positive 

 statements made in regard to the change of colour of Lepus americanus 

 by moult alone can apply to the Variable Hares of Europe as regards 

 the autumn change. The spring change, on the other hand, appears 

 to be due in all cases which have been studied to a change of coat. 



Summarised from a physiological aspect, it would appear that 

 there are two conditions of the animal's body, in one of which white, 

 in the other pigmented, hairs are produced. 



That this is so cannot indeed be doubted in view of the experiments 

 of Professor Halliburton and Drs. Brodie and Pickering.^ These 

 investigators have shown that the condition of the intravascular blood 

 varies in animals which, like the Arctic Hare, are sometimes white, 

 and at other times in a pigmented state. Further, that the composition 

 of the blood of an animal which has undergone winter whitening is 

 similar to that of a permanent albino. The presence or absence of 

 pigment is then but the external evidence of changes occurring 

 internally in an animal possessing a varying metabolism. So the 

 white hairs of Arctic animals must be regarded as due to a cause 

 similar to that which brings about absence of pigment in albinos, and, 

 almost certainly also, in aged animals. 



And, since it has been shown that Arctic animals possess a vaiying 

 metabolism, it seems most reasonable to suppose that the vital 

 changes reach their lowest point at the same season as in the human 

 race, for which physicians accept a metabolism at its lowest in autumn. 

 That is to say, metabolism is lowest just at about the very time of 

 the change from brown to white (Dr. J. Netton Eadcliffe in Quain's 

 Medical Dictionary, p. 114). 



It is at this very season that there comes the shock to the system 

 of the onset of the cold of the Arctic winter, and, as is well known, 



1 Journal of Physiology, xvi., p. 135, 1894 ; xviii., p. 285, 1895 ; xx., pp. 310 

 to 315, 1896. 



