664 THE PLEISTOCENE PERIOD 



best balance (i) between the vegetation and the soils and climate on 

 which it depends, (2) between plants and herbivorous animals, and 

 (3) between the carnivorous animals and the herbivores on which 

 they prey. 



To-and-fro migration. An important biological effect of the 

 ice-sheets on life, was forced migration. With every advance of 

 the ice, the whole fauna and flora of the region affected had to move 

 on in front of it, or die. The arctic species along the ice border 

 crowded upon the sub-arctic forms just south of them, these in turn 

 crowded upon the cold-temperate species beyond, and so on. It 

 is not unlikely that even the tropical zones were somewhat narrowed. 

 During the interglacial epochs, migrations were reversed. As the 

 advances and retreats of the ice caused migrations back and forth, 

 every organism was obliged to adapt itself to a new zone, to migrate, 

 or to die. There appear to have been four or five such to-and-fro 

 migrations in America and Europe, and the extent of the migrations 

 was several hundred miles, and in some cases perhaps one to two 

 thousand miles. During some of the interglacial epochs, the life 

 of middle latitudes indicates a climate milder than the present, and 

 this implies that the ice-sheets were reduced at least as much as 

 now. During some of the interglacial epochs, northern lands seem 

 to have supported as many plants and animals as now. Geological 

 evidence warrants the belief that at least some of the interglacial 

 intervals were long enough, and their climates warm enough, to per- 

 mit a complete northward return of the life which was forced south 

 during glacial epochs. 



Relics of glacial migrations. Significant evidence of the to-and- 

 fro migrations of the period is found in the life of the higher moun- 

 tains within or near the borders of the once glaciated areas. When 

 the ice was near these mountains, arctic life only could have existed 

 there. As the ice retired to the north, the arctic life of the surround- 

 ing lowlands moved northward also, and life from the temperate 

 zone came on to take its place; but in the mountains the arctic life 

 still found congenial conditions by moving up to higher and higher 

 levels as the climate became warmer. In this way arctic life be- 

 came isolated in the high mountains. Plants, insects, and small 

 mammals whose kin now live in the arctic zone, remain to this day 

 in some of the higher parts of the northern Appalachians, and the 

 same point is still more strikingly illustrated in the Alps. 



Life of interglacial epochs. By far the larger part of the fossils 



