THE PLEISTOCENE OR GLACIAL PERIOD. 485 



new species or became extinct. The same appears to have been true 

 of the insects. Interpreted in the light of the above considerations, 

 there seems warrant for the view that the ice invasions wrought a 

 very serious depression in the life of the globe. It is scarcely possible 

 to avoid the conviction that, at the height of glaciation, the sum total 

 of life on the globe was very greatly reduced. It is probable that 

 even the re-expanded life of to-day is appreciably inferior in abundance 

 to that of the middle Tertiary. Our era is probably one of relative 

 impoverishment, and what is perhaps more important, it is probably 

 a period of relatively poor adjustment of life to life, and of life to phys- 

 ical environment. It is improbable that, in the process of recovery 

 of the millions of square miles denuded of life by the ice-sheets, there 

 has yet been worked out the best balance between the vegetative life 

 and the soils and climatic conditions on which it is dependent, between 

 the herbivorous animals and the plants on which they are dependent, 

 and between the carnivorous animals and the herbivores on which 

 they prey, together with all the complicated sub-adjustments that 

 are involved in a well-adjusted peopling of the earth. 



To-and-fro migration. — 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 imme- 

 diately adjacent to the ice border crowded upon the sub-arctic forms 

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

 perate 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 

 succeeding deglaciation of the interglacial stages, a reversed migration 

 followed. Present evidence 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 northward swing of these movements 

 was several hundred miles in extent, in some cases perhaps one to 

 two thousand miles. Some of the interglacial epochs saw a northward 

 extension of mild-temperate forms greater than that of to-day, from which 

 it is inferred that the interglacial 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 



