108 HUMAN HISTOEY 



which it is supposed that the later subdivisions of the Cainozoic 

 period have covered, it may be noticed that Penck estimates the 

 length of the Pliocene and Miocene periods at about two and 

 three million years respectively. 



It is the dating of the subdivisions of the Pleistocene epoch 

 that is of interest here. This era is also known as the glacial 

 epoch. During this period large areas both in the northern and 

 southern hemispheres were covered by the formation of glaciers 

 or by the advance of previously- existing glaciers. It is now 

 almost universally admitted that in Europe at least there were 

 four separate advances of the ice. After reaching on each occasion 

 a point of maximum extension, the ice retreated, and there were 

 thus three inter-glacial or genial epochs, while the period since 

 the fourth glaciation is known as the Eecent period. The limit 

 of the extension of the ice varied in each glaciation, and was at 

 its maximum in the second glaciation. These separate glaciations 

 and intervening genial periods serve to subdivide the Pleistocene 

 era, and w r hen fossil and cultural remains are discovered an 

 attempt is made to ascertain in which of these subdivisions they 

 occur. 



Nothing more than guesses can be made as to the length of the 

 glacial period as a whole and of the subdivisions, and these guesses 

 differ very widely. Osborri gives the following list of estimates : l 



1863. C. Lyell . . . 800,000 years 



1874. J. D. Dana . . . 720,000 



1893. C. D. Walcott . . 400,000 



1893. W. Upham . . . 100,000 



1894. A. Heim . . . 100,000 

 1900. W. J. Sollas . . 400,000 



1909. A. Penck . . . 520,000-800,000 years 

 1914. J. Geikie . . . 620,000 (minimum) years 



Osborn adopts the more conservative estimate of Penck for 

 the duration of the whole period, and gives a subdivision of the 

 period, which is as follows : 2 



1 Osbora, Old Stone Age, p. 22. 



* Ibid., p. 23. It is well to lay stress on the fact that all these figures are 

 little more than guesses founded on various observations such as the rate 

 of deposition of sediment by rivers. No conclusions with regard to evolution 

 can be drawn from the supposed length of the whole period or from the supposed 

 length of the subdivisions. Nevertheless the relative length of the different sub- 

 divisions as estimated has in all probability some approximation to fact, and 

 thus, whatever may be the real length of the period, the time which elapsed 

 between, say, the beginning of the Pleistocene and the middle of the third genial 

 epoch was at least twice and perhaps three times longer than that which has 

 elapsed since the beginning of the latter epoch to the present day. 



