species spread principally from three centres
over Europe--viz., from the Lusitanian, Alpine, and the Balkan centres.
The southern element of the British fauna is therefore composed of
animals which have originated in these three centres, and in Central and
Southern Asia. The Balkan species have been included with those coming
from the latter centre under the term "Oriental" migration. The sixth
chapter is devoted to it, whilst the Lusitanian and Alpine migrations
have each a chapter to themselves.

The Arctic Hare is, as I have already mentioned, one of the mammals of
the northern element of the British fauna. It is now confined to the
mountains of Scotland and the plain and mountains of Ireland. But in
former times it had a wider range in the British Islands. The Stoat is
another distinctly northern mammal. It occurs with us, as Messrs. Thomas
and Barrett-Hamilton have pointed out, in two distinct varieties or
species, the one being confined to Great Britain, the other to Ireland.
As I shall explain more fully later on (p. 135), I have reasons to
believe that the Irish Stoat came from the Arctic Regions as a northern
migrant, but that the English Stoat, on the other hand, reached England
with the Siberian fauna from the east. A third northern animal, now
extinct in the British Islands, is the Reindeer. It is supposed to have
died out in these countries not very many centuries ago, and records
have been handed down to us that it still inhabited Scotland as late as
the thirteenth century. Like the Stoat, it occurred in two well-known
varieties, distinguished from one another by the shape and form of the
antlers. In the English pleistocene deposits the remains of both kinds
are met with mingled together, whilst in Ireland only one of them has
been found. The explanation of this case is similar to that of the two
stoats. One of the varieties, which we may call the northern one, came
to us from the Arctic Regions; the second wandered to the British
Islands at a later period, when Ireland had probably become separated
from England. It was therefore unable to penetrate so far west.

One of the most familiar examples of a northern British bird is the Red
Grouse (_Lagopus scoticus_). By most authorities it is looked upon as a
species distinct from the Scandinavian Willow Grouse (_Lagopus albus_),
but except in colour it is indistinguishable from it, and the eggs are
identical. The whole genus _Lagopus_ is a distinctly Arctic o