212 MOSTLY MAMMALS 
convey an altogether different meaning, the term “ dichroic” 
would be more appropriate, seeing that the difference 
between the two phases is solely one of colour, and has 
nothing to do with shape or structure. Using, then, the 
term ‘dimorphism ” as indicative of the existence in one 
animal of two distinct colour-phases totally unconnected 
with either locality or season, the Arctic fox appears to be 
the only mammal to which this designation can be 
properly applied. 
The reason for this remarkable dimorphism in the Arctic 
fox is hard indeed to discover, and no satisfactory explana- 
tion of the puzzle appears hitherto to have been offered. 
It is almost unnecessary to say that the reason why 
Arctic and sub-Arctic animals turn white in winter is that 
they may be as inconspicuous as possible in their environ- 
ment of snow and ice. And if blue foxes were met with 
only in countries where snow lies but a short. time in 
winter, while white ones occurred solely in more northern 
lands, some clue to the puzzle might be forthcoming. But, 
as a matter of fact, this is not the case. 
The distribution of the Arctic fox is circumpolar, ex- 
tending in the New World about as far south as latitude 
50°—that is to say, nearly to the southern extremity of 
Hudson Bay—and in the Old World to latitude 60°, or, 
approximately, to the latitude of Christiania and the Shet- 
land Isles. Northwards the species extends at least as far 
as Grinnell Land. 
In Iceland all the Arctic foxes appear to belong to the 
blue phase, and as that island is far to the south of many 
portions of the habitat of the species, it might be thought 
that this is the reason why the white phase is unrepre- 
sented there. But that island is far north of the line 
where the mountain-hare and the stoat begin to assume 
