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. B}^ 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. 



Evidence of intei^glacial 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 antiqnus), a rhinoceros {Rhinoceros etruscus), the urus 

 {Bos primigenins), the red deer, {Cervus elephas) and the cave- 

 bear, {Ursus spelmns)] and there were also abundance of 

 insects.^ 



In Sweden also there are tw^o tills," the lower one having 

 been in places partly broken up and .denuded before the upper 

 1 Heer'8 Primceval World of Switzerland. Vol. IL, pp. 148-168. 



