340 THE BIOLOGY OF THE SEASONS 



or ermines are comparatively rare in England. The infer- 

 ence is that the changes which bring about the whiteness 

 require at least the stimulus of a considerable degree of cold. 

 It does not by any means follow that the cold is the direct or 

 mechanical cause of the whiteness. 



To the question, What actually takes place when the white 

 dress is put on, a fairly secure answer can be given in some 

 cases. But in other cases we have not as yet sufficient data. 

 It is well known that the stoat or ermine (Mustela erminea), 

 which is brownish -red in summer, usually becomes a beautiful 

 white in winter, all but the black tip of the tail. How is this 

 effected ? The older naturalists, such as Bell, who wrote on 

 British Mammals, believed that the brownish-red hairs lost 

 their pigment and became white, which is not in any way 

 improbable. But Professor William MacGillivray was one 

 of the first to doubt the accuracy of this view, pointing, for 

 instance, to a specimen caught in December, which showed 

 a mixture of white and brownish-red hairs. " The hairs of 

 the latter colour were not in the least degree faded, and those 

 of the former were much shorter, and evidently just shoot- 

 ing ; so that the change from brown to white would seem to 

 take place by the substitution oj new white hairs jor those oj the 

 summer dress." This view has been confirmed by the very 

 careful investigations of Professor G. Schwalbe. 



Several instances of stoats of a brown colour patched with 

 white, in which the white hairs were of the same length as 

 the brown, led MacGillivray to think that " sometimes the 

 brown hairs themselves, on the application of intense cold, 

 become whitened " ; and of this as an exceptional occurrence 

 there is some modern corroboration. For in the case of the 

 mountain hare, in which, according to MacGillivray, the 

 white winter hairs are due to fresh growth, the investigation 

 of von Loewis leads to the conclusion that brown hairs may 

 sometimes be changed, in situ, into white ones, 



