PLEISTOCENE CLIMATE AND MAMMALIA. 
395 
waves, and a second period of g'laciers set in, traces of which 
occur abundantly in Wales, Scotland, Ireland, and even as 
far south as Dauphine and Auvergne. They were, how- 
ever, of far less extent than those which preceded them, 
occupying isolated areas instead of forming one continuous icy 
covering to the country. Such as this is a brief resume of the 
glacial phenomena : 1 . As the pleiocene temperature was 
lowered, the glaciers crept down from the tops of the moun- 
tains, until at last they formed one continuous ice sheet, 
moving resistlessly over the smaller hills and valleys to the 
lower grounds, and the first glacier period set in. 2. Then 
followed the period of depression beneath the sea. 3. And, 
lastly, on the land re-emerging from the sea, the second glacier 
period set in. The climate during the marine depression must 
obviously have been milder than that of either of the glacier 
periods, because of the moderating effect of the presence of a 
stretch of sea. 
What is the precise relation of the Pleistocene mammals 
to these two glacier periods ? Did they invade northern and 
central Europe during the first or the second, before or 
after, the marine submergence indicated by the “ middle 
drift?” We might expect, a 'priori , that as the temperature 
became lowered the northern mammalia would gradually 
invade the region occupied before by the pleiocene forms, and 
that the reindeer and the mammoth would gradually sup- 
plant the Cervus ardens and the Elephas meridionalis. 
Traces of such an occupation would necessarily be very rare, 
since they would be exposed to the grinding action both of the 
advancing glacial sheet, and also subsequently to that of the 
waves on the littoral zone during the depression and re-eleva- 
tion of the land. At the time also that the greater part of 
Great Britain was buried under an ice sheet, they could not 
have occupied that region, although they may have been, and 
most probably were, living in the districts further to the south, 
which were not covered by ice. The labours, however, of Dr. 
Bryce and others proved that one at least of the characteristic 
Pleistocene mammalia— the mammoth as well as the reindeer 
— lived in Scotland before the deposit of the lower boulder- 
clay ; while Mr. Jamieson has pointed out that they could not 
have occupied that area at the same time as the ice, and there- 
fore must be referred to a still earlier date.* The teeth and 
bones discovered in the ancient land surface at Selsea also 
very probably indicate that the mammoth lived in Sussex 
* For account of these discoveries, see 11 Trans. Geol. Soc. Glasgow,” 
vol. i. part 2 ; 11 Quart. Geol. Journ.,” vol. xxi. pp. 161 et seq. and pp. 204 et seq. 
I am also indebted to Mr. James Geikie for valuable information on the 
subject. 
