PRESENT-DAY DISTRIBUTION 



the whole question becomes more simple. The 

 geological records, even of the Northern Hemisphere, 

 are notoriously imperfect but as investigations pro- 

 ceed many links are forged and abysmal chasms 

 bridged. The human mind, collectively or individ- 

 ually, will never achieve the infinite but it may learn 

 enough to* explain much intelligently. 



If we are in the least degree to understand 

 the present-day distribution of plants, and especially 

 the isolation of groups of trees like for instance the 

 Honey-locust (Gleditsia), and Sweet-gum (Liquid- 

 ambar), which occur in Asia Minor, China, Japan, and 

 eastern North America and each separated by thou- 

 sands of miles of land and sea, it is necessary that we 

 try and picture some of the changes time has wrought 

 in the climate of the Northern Hemisphere. Geolo- 

 gists are pretty well agreed that the two great oceans, 

 Atlantic and Pacific, have not changed much in the 

 aeons of time since this earth began to cool. Seas, 

 plains, mountain ranges, and large areas of land 

 have, however, changed vastly though probably the 

 depressions and elevations have maintained a fairly 

 stable equilibrium — a sort of compensation balance. 



The Tertiary period, that is the geological era imme- 

 diately preceding the present, was one of great dis- 

 turbances and the folding of the earth's crust, due to 

 internal cooling and consequent contraction, made 



17 



