BRITISH FOSSIL SPECIES 135 



distributed and characteristic Late Pleistocene form; it is 

 specialised for fossorial habits, and is no doubt nearly related to 

 the living members of the A. scherman group. Of the species of 

 Microlus, M. corneri is of special interest as being the forerunner 

 of the peculiar M. orcadensis and its representative in the Channel 

 Islands, M. sarnius. 



e. Third Terrace of the Thames and its tributaries (typical 

 development in the Lea Valley at Ponder's End and Angel Road. 

 Three species are at present known to occur : — 



"fDicrostonyx henseli Hinton. 

 fMicrotus anglicus Hinton. 

 *Microtus arvalis group. 



In deposits of this stage remains of Dicrostonyx henseli occur 

 in great abundance, all other species being extremely rare or 

 absent. Since the publication of Mr. S. Hazzledine Warren's 

 accounts of the Third Terrace in the Lea Valley ^ it has been 

 generally recognized that the terrace in question represents a 

 cold period. The discovery of a cold fauna and flora at that 

 horizon was predicted by me ^ ; and in my opinion the Third 

 Terrace marks the opening of the major glaciation of this country. 

 In surveying the Pliocene and Pleistocene succession in Britain 

 it is not until we reach late Pleistocene times that we find any 

 evidence at all of a " cold fauna or flora " or any real evidence of 

 glaciation ; it is not possible to frame a glacial theory that 

 will fit all the facts, be they physiographical, stratigraphical, or 

 palaeontological, unless we are prepared to place the " major 

 glaciation " at this comparatively late moment. 



Many deductions as to former climatic conditions have been 

 and no doubt will continue to be made from the presence or 

 absence of particular species or groups of species in successive 

 geological horizons. Such deductions should, however, be made 

 with the greatest caution. In my opinion no reliable evidence 

 of changes of climate is afforded by the fossil mammals of countries 

 now temperate. Very few species are unable to exist in temperate 

 conditions ; their presence or absence depends upon access, food, 

 shelter, and competition, rather than upon climate. Among 

 Microtinae, Microlus nivalis, the Snow Vole, buried for ten months 

 in a year under Alpine snows, and Dicrostonyx, the " warmth- 

 hater " of Hcnsel, have been favourite subjects for geological 

 comment and speculation. The presence of their remains in 

 abundance on the plains of temperate Europe has been regarded 

 again and again as a strong proof of former severe climatic con- 

 ditions. But M. nivalis still lingers on in the hot lowlands of 

 southern France, and Dicrostonyx lives in the by no means Arctic 



» S. Hazzledine Warebn, Q.J.G.S., 68, p. 213, 1912; and 71, p. 164, 

 1916. 



' HiXTON, Proc. Geol. Assoc, 21, p. 503, 1910. 



