308 Proceedings of the Royal Irish Academy. 



Metchnikofi/ who in the case of old men and dogs saw the phagocytes 

 passing from the medullary to the cortical layers of whitening hairs. 

 These phagocytes ingest the pigment granules, and remove them into 

 the body — a process which, as Professor Metchnikoff believes, '* can be 

 classed under the general law of atrophy of solid parts of the 

 organism." And that the organism is at this time economical of its 

 resoui'ces, and unwilliag to waste them peripherally, is probably 

 indicated by the fact that the silkiness of the winter hairs of animals 

 inhabiting cold countries indicates a fineness of texture, that is to say, 

 less material is appropriated for their manufacture in proportion to 

 their length than for the summer hairs. Since the animal organism 

 has power to recall pigment fi-om its hair, it matters not whether or 

 no the physiological causes of winter whitening culminate at the time 

 of a moult. Once the required condition prevails, new hairs will 

 grow, and already existing hairs will rapidly become white. 



As to the reverse process, the recolouration of the coat, years of 

 study have failed to supply me with an instance of its occurrence 

 without a moult : so that the conclusion seems hardly avoidable that 

 the hairs, once whitened, are dead to further changes of colour". 

 Thus is explained the curious fact that in the mild climate of the district 

 where I write, such winter whitening as occurs in the Hares (and 

 it is sometimes considerable in extent) remains in force, no matter 

 what the weather may be, until the spring moult. As this moult does 

 not take place until the spring has well advanced, those Hares which 

 have undergone the most complete winter change are for some little 

 time incongruously conspicuous in the flowery meadows of the south of 

 Ireland, while the April and May sunshine lights up their Arctic 

 livery. At the time of the spring moult, the physiological causes 

 which led to the whitening of the previous autumn having now passed 

 away, vital change being now at its high-water mark, and fat, and 

 with it pigment, available for constructive purposes throughout the 

 body, the new coat (or, in the case of birds, the plumage) comes up 

 of the pigmented summer tint. 



But it is not fail' to regard as typical of its kind the cycle of winter 

 whitening as observed in England or Ireland. Here at the southern 

 limit of the conditions which have called it into existence, the process 

 is complicated by numbers of contradictory factors, the resultant of 

 which is a considerable modification and obscuring of the typical 

 phenomena. In the Arctic regions these are nearly uniform. 



iProc. Eoy. Soc, London, vol, Ixix., p. 156, 1901. 



