METHODS — GEOLOGICAL 27 



value of the testimony of fossils as to climatic conditions. The 

 problem is one of great complexity, for the Pleistocene was not 

 one long epoch of unbroken cold, but was made up of 

 Glacial and Interglacial ages, alternations of colder and milder 

 conditions, and some, at least, of the Interglacial ages had a 

 climate warmer than that of modern times. Such great 

 changes of temperature led to repeated migrations of the 

 mammals, which were driven southward before the advancing 

 ice-sheets and returned again when the glaciers withdrew 

 under the influence of ameliorating climates. Any adequate 

 discussion of these complex conditions is quite out of the 

 question in this place and the facts must be stated in simplified 

 form, as dealing only with the times of lowered temperature 

 and encroaching glaciers. 



The plants largely fail us here, for little is known of Glacial 

 vegetation, but, on the other hand, a great abundance of the 

 fossil remains of animal life of that date has been collected, and 

 its testimony is quite in harmony with that afforded by the 

 ice-markings and the ice-made deposits. Arctic shells in the 

 marine deposits of England, the valley of the Ottawa River 

 and of Lake Champlain, Walruses on the coast of New Jersey, 

 Reindeer in the south of France, and Caribou in southern New 

 England, Musk-oxen in Kentucky and Arkansas, are only a 

 few examples of the copious evidence that the climate of the 

 regions named in Glacial times was far colder than it is to-day. 



I have thus endeavored to sketch, necessarily in very 

 meagre outlines, the nature of the methods employed to re- 

 construct the past history of the various continents and the 

 character of the evidence upon which we must depend. Should 

 the reader be unconvinced and remain sceptical as to the possi- 

 bility of any such reconstruction, he must be referred to the 

 numerous manuals of Geology, in which these methods are 

 set forth with a fulness which cannot be imitated within the 

 limits of a single chapter. The methods are sound, consisting 

 as they do merely in the application of ''systematized common 



