62 MOSTLY MAMMALS 
it would be the basal half which was white, and the 
terminal half which retained its natural colouring—in other 
words, precisely the reverse of the condition obtaining in 
Sir John Richardson’s lemming, thereby affording further 
presumptive evidence as to the abnormal condition of the 
change in that animal. 
As a matter of fact, however, those of us who have 
reached an age when silver hairs have begun to make 
their appearance among the brown can easily satisfy them- 
selves that such hairs are white throughout their entire 
length, and that a hair half white and half brown is quite 
unknown. From this we infer that the change from brown 
to white takes place in human beings by the gradual 
shedding of the dark hairs and their replacement by new 
ones from which pigment is entirely absent. So that 
normally there is no such thing as bleaching of individual 
hairs. The change is, indeed, precisely similar to the one 
which takes place at the approach of winter in mammals 
that habitually turn white at that season, with the exception 
that, as a general rule, it is extremely slow and gradual, 
instead of being comparatively rapid, and also that the white 
hairs differ from their dark predecessors solely by the 
absence of colouring-matter. Unfortunately, there is no 
subsequent replacement of the white hairs by dark ones! 
The fact that the change from brown to white in the 
mountain hare (Lepus timidus) is really due to a change 
of coat and not to bleaching was known at a very early 
period to the English naturalist Pennant; and the exist- 
ence of this change was likewise recognised by Macgillivray. 
It was not, however, till Dr. J. A. Allen, in a paper on 
the colour-change in the North American variable hare 
published in the Bulletin of the American Museum of Natural 
History for 1894, demonstrated by actual experiment the 
