PLEISTOCENE CLIMATE AND MAMMALIA. 
393 
of the reindeer. The North American mammalia also vary in 
their range according to the climate, as Sir John Franklin 
found, to his cost, when he was travelling over the barren 
grounds from the shore of the Arctic Sea. In both America 
and Siberia there is a zone of debateable ground, in which the 
mammals of the Arctic and temperate provinces are continu- 
ally oscillating to and fro, according to the seasons. And in 
this their skeletons could not fail to be mixed together in the 
deposits of the rivers. The musk sheep, for instance, which 
in Herne’s day, a.d. 1772, lived near Fort Churchill, has now 
left that district to be occupied by the elk and the wapiti. 
There can be no doubt that the mixed character of the 
Pleistocene fauna in Britain and Central Europe is due to a 
similar oscillation to and fro of the animals according to the 
seasons ; and when we consider the geographical position of 
that area at the time (PI. LXXVIII.), we can see at once how 
the mammalia occupying it must necessarily have been mixed. 
The land stretched continuously without any impassible 
barrier northwards and eastwards to the present home of the 
reindeer in Euro-Asia ; while, on the other hand, it reached 
southwards over a considerable portion of what is now the 
Mediterranean, almost, if not quite, touching Africa. The 
winter cold and the summer heat of so great a mass of 
land must necessarily have been more severe than now, when 
the Mediterranean occupies a far wider area, and when the 
Atlantic and the Baltic and the North Sea have considerably 
diminished the area of the land. It is therefore by no means 
to be wondered at that a southern animal, such as the hippo- 
potamus, should have wandered northwards and westwards as 
far as the latitude of Yorkshire, and it is worthy of note that 
this is the extreme northern limit of the range of the animal. 
On the other hand, during the severity of winter the reindeer 
and the musk sheep descended southwards, and occupied the 
area which they deserted at the approach of summer. Such, 
in my belief, is the explanation of the mixed character of the 
Pleistocene fauna ; it arises partly from the climatal extremes 
which must result from the extension of the European continent 
over what is now sea. The continuity of land also northwards 
and southwards afforded room for the swinging to and fro of the 
northern and southern forms of life. When that continuity 
was broken the animals would be cut off from their bases of 
retreat, and disappear from a region in which the climate was 
passing from a continental to an insular condition. It must, 
however, be admitted that the enormous preponderance of 
northern over southern animals in Pleistocene Europe implies 
a great severity of winter cold, while the comparatively few 
remains of southern animals show that they were rarely able 
