Ch. X.] FLUCTUATIONS OF CLTMATE. 131 



Thames, where the three pachyderms last enumerated are found 

 together, a fossil shell, Cyrena fluminalis, is abundant, which no longer 

 lives in any European river, but still inhabits the Nile and parts of 

 Asia. "With it, in the same sand and gravel, the TJnio littoralis occurs, 

 now extinct in Britain, but still living in the Seine and Loire in 

 France." It may be contended that when the Cyrena fluminalis 

 abounded in the Thames, the hippopotamus may have been suited to 

 the same climate, just as the same mollusk and the living hippo- 

 potamus now coexist in the Nile. W 7 e may doubtless imagine that 

 during the countless centuries which may have passed away since the 

 glacial epoch, there have been oscillations of temperature, in the 

 course of which certain members of a more southern fauna migrated 

 northwards, and then retreated again when a succession of less genial 

 seasons prevailed, while other migrations in an opposite direction 

 took place whenever there was a change from a warmer to a colder 

 climate. 



In the valley of the Somme the rude flint tools before mentioned, 

 page 116, have been found at Menchecourt, near Abbeville, associated 

 with the Cyrena fluminalis, and with the Hippopotamus major. 

 These were met with in the lower level post-pliocene gravel, and may 

 be referable, as Mr. Prestwich has suggested, to a period when the 

 climate was somewhat wanner than that of the higher level drift of 

 this same valley. It is in that higher and older drift at St. Acheul, 

 near Amiens, that flint implements have been found in the greatest 

 number, together with the bones of the elephant and other post- 

 pliocene quadrupeds, so that man, must have existed through several 

 successive phases of the geography and climate of that region in 

 prehistoric times. 



In 1863, several individuals of the Greenland lemming, and several 

 of a new species of Spermophilus, an Arctic type allied to the mar- 

 mot, were found by Dr. Blackmore in the ancient alluvium of the 

 Wiley near Salisbury, in lower-level drift, rising about thirty feet 

 above the present water meadows. They were associated with the 

 mammoth, tichorine rhinoceros, cave hyama, reindeer, and many other 

 mammalia, probably suited, like them, to a cold climate. In the im- 

 mediate vicinity occurs a higher level gravel, ninety feet above the 

 Wiley, from which flint implements, much rolled and resembling 

 some of those at Amiens, have been obtained. After examining the 

 spot, I agree with Dr. Blackmore, that these flint tools, and the gravel 

 in which they are embedded, are older than the deposits containing 

 the extinct mammalia, so that in this instance we cannot suppose, 

 as in the case of Menchecourt above alluded to, that the fossils of 

 the more modern or lower-level deposit indicate a more genial 

 climate. 



Nearly all the known post-pliocene quadrupeds have now been 

 found either in valley drifts or cave deposits in England or on the 

 Continent, accompanying flint knives or hatchets in such a way as to 



