509 
Again ; as its remains have never been found in high 
northern latitudes, it may more probably have been only 
a periodical visitor in England, France, and Germany, and not 
a dweller throughout the year, like other herbivores — as the 
elephant, rhinoceros, Irish elk, bison, reindeer, and its head- 
quarters have been confined to the shores of the Mediterranean 
and the north of Africa, where its remains have occurred near 
Constantine, in Algeria. Again, the occurrence of the hippo- 
potamus may be accounted for in a somewhat different man- 
ner ; for instance, as it appears self-evident that the general 
climate of Britain, during the post-glacial epoch, has been 
more severe than at present, a period or periods of some 
length may have intervened, while England and Ireland 
formed a portion of the European continent, when the 
climate may have been less severe, and the rivers free 
from ice throughout the year. This inference is somewhat 
strengthened ; but the fact that the remains of the hippo- 
potamus are most frequently associated with those species of 
elephant and rhinoceros — i.e., Elephas antiquus and Rhino- 
ceros leptorhinus — which not only occur in this hemisphere, 
but have had a southern or tropical, rather than a northern 
range ; and as certain plants belonging to the Lusitanian flora 
are found on the west coast of Ireland, it is only reasonable 
to suppose that their migration from a southern land has 
occurred along a coast line since the glacial epoch, which 
would have exterminated them by its severity. Is it not 
probable, therefore, that at the period which favoured the 
migration of the flora, certain southern forms of mammals 
have migrated also ? 
The evidence of a gradual increase of temperature in 
France and Germany during the Historical period appears 
perfectly certain. From Diodorus Siculus we learn that the 
Rhine and the Danube, during the first four centuries, were 
frequently frozen over in the winter; and Csesar mentions 
