CHAP. VII THE GLACIAL EPOCH 117 



of the evidence on which the existence of the glacial epoch 

 depends. There is perhaps no great conclusion in any 

 science which rests upon a surer foundation than tliis ; and 

 if we are to be guided by our reason at all in deducing the 

 unknown from the known, the past from the present, we 

 cannot refuse our assent to the reality of the glacial 

 epoch of the northern hemisphere in all its more important 

 features. 



Effects of the Glacial Epoch on Animal Life : Warm and 

 Cold Periods. — It is hardly necessary to point out what an 

 important effect this great climatal cycle must have had 

 upon all living things. When an icy mantle crept gradu- 

 ally over much of the northern hemisphere till large 

 portions of Europe and North America were reduced to 

 the condition of Greenland now, the greater part of the 

 animal life must have been driven southward, causing a 

 struggle for existence which must have led to the exter- 

 mination of many forms, and the migration of others into new 

 areas. But these effects must have been greatly multiplied 

 and intensified if, as there is very good reason to believe, 

 the glacial epoch itself — or at least the earlier and later 

 phases of it — consisted of two or more alternations of warm 

 and cold periods. 



The evidence that such was the case is very remarkable. 

 The " till,'' as we have seen, could only have been formed 

 when the country was entirely buried under a large ice- 

 sheet of enormous thickness, and when it must therefore 

 have been, in all the parts so covered, almost entirely 

 destitute of animal and vegetable life. But in several 

 places in Scotland fine layers of sand and gravel with beds 

 of peaty matter, have been found resting on " till " and 

 again covered by " till." Sometimes these intercalated 

 beds are very thin, but in other cases they are twenty or 

 thirty feet thick, and in them have been found remains of 

 the extinct ox, the Irish elk, the horse, reindeer and 

 mammoth. Here we have evidence of two distinct periods 

 of intense cold, and an intervening milder period suffi- 

 ciently prolonged for the country to become covered with 

 vegetation and stocked with animal life. In some districts 

 borings have proved the existence of no less than four 



