CHAP, viri.] NEOLITHIC INHABITANTS OF BRITAIN, ETC. 265 



Comparative Duration of Pleistocene and Prehistoric 



Periods. 



We may obtain a rough approximation to the relative 

 length of the Pleistocene and Prehistoric periods from 

 the fact that the valleys were cut down by the streams 

 flowing through them ; in the former, sometimes as much 

 as a hundred feet, while the work done by the rivers 

 during the latter is measured by the insignificant 

 fluviatile deposits close to the adjacent stream. 1 It may 

 therefore be concluded that the former period was 

 beyond all calculation longer than the latter. The latter, 

 however, may. have been of very considerable length, 

 since it includes a series of changes in the fauna, and a 

 series of invasions of different races of men into Europe, 

 which, if measured by similar changes recorded in 

 history, must have required the lapse of many centuries. 

 In dealing with these questions it is only possible to 

 grasp the relative duration, for the measurement of time 

 absolute in terms of years outside the reach of history 

 is beyond our power. We do not know the length of 

 the interval separating any two events not recorded in 

 history, nor are we possessed of any natural chronometer 

 by which to fix a date in the historical sense. We are 

 dealing merely with time relative, and not time absolute. 



Neolithic Inhabitants of Britain and Ireland. 



The great changes in the fauna and geography of 

 Britain, at the close of the Pleistocene age, render it 



the People, 1872, p. 19 ; Reports of Kent's Cavern Committee; Brit. Ass. 

 Reports, 1865 to 187 8. 



1 Cave-hunting, p. 267 ; Evans, Ancient Stone Implements, c. xxv. 



