THE PLEISTOCENE OR GLACIAL PERIOD 915 



zones were somewhat shifted, and the zones themselves narrowed. 

 During the interglacial epochs, migrations were reversed. As the 

 zones were shifted back and forth alternately by the advances 

 and retreats of the ice, every organism was under special stress to 

 adapt itself to a new zone, to migrate, or to die. There appear to 

 have been four to six such to-and-fro migrations in America and 

 Europe, and the swing of these movements was several hundred 

 miles, and in some cases perhaps one to two thousand miles. Dur- 

 ing some of the interglacial epochs, the life of middle latitudes 

 indicates a climate milder than the present, and hence 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. This carries the conclusion that the 

 migratory swing in these more pronounced cases was some 2,000 

 miles in North America, and more than 1,000 miles in Europe. As 

 indicated in the physical description, other geological evidence 

 warrants the belief that the interglacial intervals were long enough 

 to permit a complete northern return of the life which was forced 

 south during glacial epochs, and the fossil evidence supports the 

 conclusion that the climates were congenial enough to invite it. 



The forced migrations must, in their nature, have been peculiarly 

 effective in causing a severe struggle for existence. Forms pre- 

 viously specialized to meet local conditions were put to a most 

 adverse test, for the invading ice forced every form within the 

 glaciated area to move on, while the fringing zones of depressed 

 temperature encircling each ice-sheet, forced plant and animal life, 

 even beyond the ice border, to seek new fields. An incidental 

 result of this wholesale migration was an unwonted commingling 

 of plants and animals, for every aggressive form pushed forward 

 in the van of the advancing zone, and hence came into new organic 

 environment, while every laggard fell behind, and was overtaken 

 by less reluctant migrants. 



Climatic adaptations. It has been remarked before that the 

 floras of the middle Tertiary were mixed, judged by the present 

 distribution. Types which we now regard as tropical lived in high 

 latitudes, with forms which are now boreal. So also species that 

 are now boreal lived in low latitudes then with forms which are now 



