68 THE CHICAGO ACADEMY OF SCIENCES. 
The process of the extinction and involuntary southward migration 
of life during the glacial period is thus graphically described by Dr. 
Tac aGhanberin = 
“A distinguishing feature of the effects of the ice invasions on the 
life of the glacial period in northern latitudes was an enforced oscil- 
latory 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 succeeding de- 
glaciation of the inter-glacial 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 inter-glacial epochs saw a northward extension 
of mild-temperate forms greater 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 2000 miles in North America, and 
more than 1000 miles in Europe. As indicated in the physical de- 
scription, 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 forced every form within the 
glaciated area to move on, while the fringing zones of depressed tem- 
perature encircling each ice-sheet forced plant and animal life, even 
beyond the ice border, to seek new fields and new relations, both 
1Geology, Vol. III, p. 485. 
