CHAP. VII.] 
THE GLACIAL EPOCH. 
117 
down the Rhine into the great river which then flowed up the 
bed of the North Sea, and thence up the Humber and Ouse 
into Yorkshire. By this route there would be only one 
watershed to cross, and this might probably have been marshy ; 
but we may also suppose the animals to have ascended the 
Bristol Channel after passing round a long extent of French and 
English coast (which would then have consisted of vast plains 
stretching far beyond the Scilly Isles), in which case they would 
find an equally easy passage over a low watershed from the 
valley of the Avon to that of the Trent and Yorkshire Ouse. 
A consideration of the long and circuitous journey required on 
any hypothesis, will at once convince us that it could never 
have been made (as some have supposed) annually, during the 
short hot summer of the glacial period itself; whereas the 
interglacial warm periods lasting several thousand years would 
allow for the animals’ gradual migration into all suitable river- 
valleys. Thus, the very existence of the hippopotamus in 
Yorkshire as well as in the south of England, in close associa- 
tion with glacial conditions, must be held to be a strong 
corroborative argument in favour of the reality of an inter- 
glacial warm period. 
j Evidence of interglacial warm periods on the Continent and in 
North America. — Besides the evidence already adduced from our 
own islands, many similar facts have been noted in other 
countries. In Switzerland two glacial periods are distinctly 
recognised, between which was a warm period when vegetation 
was so luxuriant as to form beds of lignite sufficiently thick to 
be worked for coal. The plants found in these deposits are 
similar to those now inhabiting Switzerland — pines, oaks, 
birches, larch, etc., but numerous animal remains are also found 
showing that the country was then inhabited by an elephant 
( Elephas antiquus), a rhinoceros ( Rhinoceros etruscus), the urus 
( Bos primigenius ), the red deer, (Cervus elephas ) and the cave- 
bear, ( Ursus spelceus'); and there were also abundance of 
insects . 1 
In Sweden also there are two “ tills,” the lower one having 
been in places partly broken up and denuded before the upper 
1 Heer’s Primaeval World of Switzerland. Vol. II., pp. 148-168. 
