14 LAND MAMMALS IN THE WESTERN HEMISPHERE 



fossils of two regions and to determine how far it is geographical, 

 due to a separation in space, or geological and caused by separa- 

 tion in time, is often a very difficult matter and requires a vast 

 amount of minute and detailed study. Once more, the princi- 

 ple involved is illustrated by the study of manuscripts. Down 

 to the time when the printing press superseded the copyist, 

 each of the nations of Europe had its own traditions and its 

 more or less independent course of handwriting development. 

 A great monastery, in which the work of copying manuscripts 

 went on century after century, became an independent geo- 

 graphical centre with its particular styles. Thus the palae- 

 ographer, like the geologist, is confronted by geographical prob- 

 lems as well as by those of change and development in general. 



In addition to the method of geologically dating the rocks 

 by means of the fossils which they contain, there are other ways 

 which may give a greater precision to the result. Climatic 

 changes, when demonstrable, are of this character, for they may 

 speedily and simultaneously affect vast areas of the earth's 

 surface or even the entire world. From time to time in the 

 past, glacial conditions have prevailed over immense regions, 

 several continents at once, it may be, as in one instance in 

 which India, South Africa, Australia, South America were 

 involved. The characteristic accumulations made by the 

 , glaciers in these widely separated regions must be contempora- 

 iieous in a sense that can rarely be predicated of the ordinary 

 stratified rocks. Such climatic changes as the formation and 

 disappearance of the ice-fields give a sharper and more definite 

 standard of time comparisons than do the fossils alone, and 

 yet the fossils are in turn needed to show which of several 

 possible glacial periods are actually being compared. 



Again, great movements of the earth's crust, which involve 

 vast and widely separated regions and bring the sea in over 

 greaj areas of land, or raise great areas into land, which had 

 been submerged, may also yield more precise time-measure- 

 ments, because occurring within shorter periods than do 



