486 Proceedings of the Royal Irish Academy. 



confined to Sontli- eastern England, many of the latest immigrants 

 from Central Europe, such as Helix pomatia, S. cartlmsiana, 

 S. canfiana, BuUminus montanus, and others. A large number of 

 species of plants might be mentioned, and also animals from almost 

 all the groups of terrestrial Invertebrates, which apparently had only 

 reached England before it became disconnected from France, and 

 which are still more or less confined to the south-eastern parts of that 

 country. Tlie majority of these arc of Central or South-eastern 

 European origin. 



Of the Invertebrates we have little or no palteontological proof of 

 the period of their migration to England. But with the Mammals it 

 is veiy different. We know that the vanguard of the Siberian migra- 

 tion reached England at the time when the Forest-bed was deposited 

 (see p. 462). Every geologist is acquainted also with the fact of the 

 extraordinary mixture of Siberian and southern types of Mammals 

 contained in this bed, as well as in the succeeding Pleistocene ones, 

 and that it has been established beyond a doubt that they must have 

 then lived together, though tlieir original homes often were situated 

 thousands of miles from one another. 



" The occuiTence," says Mr. Lydekker (57 ^i, p. 310), " of the hip- 

 popotamus in association with the musk-ox, glutton, and walrus, pre- 

 sents us with another of the puzzles which almost break the heart of 

 the paleontologist." With regard to the epoch of the English 

 caverns and brick-earths, which are by Mr. Lydekker included in the 

 Pleistocene Epoch along with the Forest-bed, he remarks (p. 300) : 

 " The most remarkable feature connected with this fauna is the apj^a- 

 rently contradictory evidence which it affords as to the nature of the 

 climate then prevalent. The glutton, reindeer, Arctic fox, and musk- 

 ox are strongly indicative of a more or less Arctic climate ; many of 

 the voles (Arvicola), picas (Lagomys) and sousliks (Spermoi)hilus), 

 togetlier with the Saiga antelope, appear to point equally strongly to 

 the prevalence of a steppe-like condition, while the hippopotamus 

 and spotted hyaena seem as much in favour of a sub-tropical state of 

 things." Prof. Dawkins (22 J, p. 113) thought that this anomaly 

 might be explained by the supposition that in the greater part of 

 Britain the winters were cold and the summers warm, as in the 

 middle of Asia and IVorth America, where large tracts of land extend 

 from the Polar region towards the Equator, and offer no barrier to 

 the swinging to and fro of the animals. In the summer time the 

 southern species would pass northwards, and in the winter time the 

 northern would swing southwards, and thus occupy at different times 



