52 SOCIAL EVOLUTION 



time in thousands and millions of years. 13 Geologists 

 divide the time of the earth's development from an un- 

 inhabitable sphere to its present state, into several great 

 epochs in accordance with the type of rock formation 

 \i-ting. The Paleozoic or Primary and the Mesozoic 

 or Secondary, cover the vast epochs when only the most 

 rudimentary forms of life existed. It is the Tertiary and 

 the (Quaternary, the periods during which the higher 

 Mammals appeared, that are of interest to us. As will 

 be seen from the diagram, the early or lower Quaternary 

 began about three-quarters of a million years ago. The 

 first t'os-il remains of prehistoric man come from the geo- 

 logical formation of this period. 



Before we may hope to have a clear understanding of 

 the antiquity of man, there is some further evidence 

 of a geological nature which we shall have to examine. 



It is important to know the main facts and theories of 

 the glacial periods, because it is the duration and fre- 

 quency of occurrence of these ice ages which give us the 

 most trustworthy evidence of the antiquity of man. If 

 we can learn the age of certain glacial deposits by meas- 

 uring their rate of formation, we are in a position to 

 say something definite as to the age of human remains 

 and implements found beneath them. It is a generally 

 accepted fact that many ages ago there were vast sheets 

 of continental ice mantling large portions of Kurope and 

 North America, just as Greenland is mantled to-day. 14 

 But it is not so generally known that the cold was not 

 constant during the age of ice. The first southward ad- 

 vance of the Arctic ice-sheet was followed by a period of 

 retreat during which temperate conditions prevailed. In 



is See figure 14 for the comparative magnitude of geologic time and the 

 age of human remains. See figure 15. 



