THE GLACIAL OR PLEISTOCENE PERIOD 217 



enough removed to be beyond the direct influence of the Arctic climate. Many 

 species found refuge in the southern part of the Appalachians where we now 

 have a mixed flora of spruce, arbor vitae, pine, hemlock, etc. South of the 

 Ohio River such deciduous plants as elms, maples, magnolias, walnuts, chest- 

 nuts, and hickories found refuge and now flourish in great abundance. 



During each successive stage of glaciation the biota was completely oblit- 

 erated in the englaciated territory, the species which were unable to migrate 

 suffering extinction. Later studies have indicated that during each inter- 

 glacial stage an extensive biota flourished, migrating from the south as the 

 territory became suitable for occupancy. This oscillation of life is thus graphi- 

 cally portrayed by Chamberlin and Salisbury. 2 



"A distinguishing feature of the effects of the ice invasions on the life of 

 the glacial period in northern latitudes was an enforced oscillatory migration 

 in latitude. With every advance of the ice, the whole fauna and flora of the 

 affected region was forced to migrate in front of it, or suffer extinction. The 

 Arctic species immediately adjacent to the ice border crowded upon the sub- 

 arctic forms next south of them, the sub-arctic forms crowded upon the cold- 

 temperate forms, and these in turn upon the warm-temperate types, and so 

 on. It is not unlikely that the limits of the tropical zones even were shifted, 

 and the torrid belt appreciably constricted. With the succeed ng deglacia- 

 ion of the inter-glacial stages, a reversed migration followed. Present evi- 

 dence seems to warrant the belief that five or six such to-and-fro migrations 

 were experienced in America and Europe, and that the southward and north- 

 ward swing of these movements was several hundred miles in extent, in some 

 cases perhaps one to two thousand miles. Some of the inter-glacial epochs 

 saw a northward extension of mild-temperate forms greate- than that of today, 

 from which it is inferred that the inter-glacial climates were milder than the 

 present, and hence that the ice-sheets were at least as much reduced as now. 

 There is in this also ground for the inference that the northern tracts were at 

 least as extensively peopled by plants and animals as they are today. This 

 carries the conclusion that the migratory swing in these more pronounced 

 cases was at least 2,000 miles in North America, and more than 1,000 miles in 

 Europe. As indicated in the physical description, the geological evidences 

 drawn from erosion, weathering, and organic accumulation warrant the belief 

 that the inter-glacial intervals were long enough to permit a complete northern 

 return, 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 bringing to bear a severe struggle for existence, and in calling into play the 

 full resources of the plastic adaptation of the life. Forms previously specialized 

 to meet local conditions were put to a most adverse test, for the invading ice 



2 Geology, III, p. 485. 



