136 PREHISTORIC EUROPE. 



a cold period would be thicker and more widely spread than 

 those which were accumulated at a time when the climate was 

 genially temperate. And again, the remains of arctic and 

 northern animals would be distributed through a wider range of 

 deposits, and might in some cases be relatively more abundant 

 than those of southern and temperate species. 



It has sometimes been urged against these views that if the 

 northern and temperate and southern species had occupied a 

 country at different periods, their bones would always occur in 

 separate and distinct deposits. We ought, it has been said, 

 to meet with beds containing remains of the northern animals 

 alone, overlying or underlying, as the case might be, strata in 

 which only the relics of southern or of temperate species 

 should occur. Now, if it were true that rivers did nothing but 

 pile one layer of gravel, sand, or mud upon another — always 

 depositing, and never rearranging what had already been laid 

 down — we might well have looked for some such arrangement 

 as that which I have referred to. Or again, if the period during 

 which one group of mammalia occupied the ground was so pro- 

 longed that the rivers were able to erode their valleys to a 

 great depth, so as to leave the slopes covered with successive 

 deposits charged here and there with animal remains, it might 

 happen that, after the old group of animals had disappeared, 

 and another group had succeeded, the last series of alluvial 

 terraces would not contain a single relic of the former, but 

 only remains of the latter. There is not the slightest reason, 

 however, for believing that the alternations of climate were 

 each of such protracted duration. Moreover, the rivers, even 

 up to the close of Pleistocene times, were able to flood their 

 valleys to a very great height, and so to bring the older gravel- 

 deposits under their influence. 



No one who shall examine any well-developed river-deposits 

 of Pleistocene age, such as those of the Thames, or of the 

 valleys in the north of France, can fail to see that they all form 

 part of one and the same series. They point to the long-con- 

 tinued action of erosion and deposition, and doubtless the river 



