NOTE ON THE OCCURRENCE OF THE ALPINE VOLE. 
191 
This problem is of course intimately connected with that concerning 
the origin of Alpine and Arctic faunas. During the time represented 
by the Middle Terrace, England was invaded by many species of 
Rodentia which could only have come from Siberia and Eastern 
Europe. As these immigrants arrived M. nivalis gradually dis- 
appeared from Britain and retreated from the plains of Central and 
Southern Europe towards the mountains. Thus in slightly newer 
deposits such as the Ightham Fissure where the new Siberian forms 
occur very abundantly the M. nivalis group and the other old southern 
forms, have completely vanished. On the lower mountain slopes the 
old forms, such as M. nivalis, were able to hold their own agaiiist 
the new comers, but the early frosts of such regions would be 
detrimental to such a species. It would learn from experience that 
security from the frost could only be had from an early covering of 
snow, and so it would quickly colonize the summits. This view is 
confirmed by many 'well known observations upon the Arctic and 
Alpine flora, and these questions are fully discussed in jny recent 
paper. A great deal of valuable evidence on these points will be found 
in Dr. Sch aril’s “ History of the European Fauna,” with whose con- 
clusions I very largely agree. I have argued that it is not necessary 
to invoke large changes in climate in order to account for the 
differences in the distribution of animals and plants which have been 
brought about since Pleistocene times, and my work among the drifts 
of the South-east of England as a geologist entirely confirms this view 
which I have based primarily upon palaeontological and distributional 
researches. 
The occurrence of the M. nivalis group at Clevedon shows that 
deposit to be not later than the deposits of the Middle Pleistocene 
period, and it supplies us with the western limit of the Pleistocene 
range of the group. 
