398 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
must perish,—over-crowding inevitably leading to death. The mor- 
tality in any one year may not have been great, but during the many 
thousands of years covered by the movements of the continental ice the 
aggregate destruction of life must have been stupendous. 
linmediately upon the close of the Glacial epoch, life began to reclaim 
the regions from which it had been so long shut out. This overflow 
released the tension under which the animals and plants had been strug- 
gling for ages and rendered the contest for existence less severe. Over- 
production had at last found an outlet, and life became possible to a con- 
stantly increasing number of individuals. Normal reproduction was 
sufficiently rapid to supply occupants for the regions made habitable 
by the slow recession of the ice, and the advance of both plants and 
animals kept pace, doubtless, with its progressive increase. But the 
species that survived to return were only in part those driven out. 
Many had been overtaken by the cold or had perished in the journey 
southward; others were driven into inhospitable regions where the en- 
vironment was not suited to their needs; others still succumbed in the 
struggle resulting from over-crowding, and some that outlived the first 
great period of glaciation perished during the second. Gilbert tells us 
that a detailed study of the ancient lake beds of the Great Basin 
‘‘shows two lacustral epochs corresponding to two glacial epochs, and 
correlates the mammalian fauna with the later half of the later Glacial 
epoch. Presumptively this date falls very late in the Pleistocene 
period.” (‘Lake Bonneville,” by G. K. Gilbert, 1890, 397.) The mam- 
malian fauna referred to comprises an elephant, an otter, two horses, 
three llamas, a deer of the genus Cervus, an ox, a gigantic sloth, to- 
gether with three species now living, namely, the coyote, beaver, and 
pocket gopher (Thomomys). No new types came in to take the place 
of those exterminated; hence we in the United States now live ina 
region deprived of many of the groups to which it gave birth, and we 
are forced to visit remote parts of the earth to see animals and plants 
that once attained their maximum development in North America, 
while others that formerly flourished here are entirely extinct. 
Not only are the pre-Pleistocene animals and plants now represented 
imperfectly and in greatly reduced numbers, but the areas at present 
inhabited by their descendants, except in the case of the Boreal forms, 
are insignificant in comparison with their former extent. It should be 
remembered that the refrigeration of the Glacial epoch has only in part 
disappeared. Jn early Pliocene times characteristic representatives of 
sub-tropical faunas and floras existed northward over much of the 
United States and Canada, and in still earlier times reached the Arctic 
Circle.* During the advance of cold in the Glacial epoch these forms 
were either exterminated or driven southward into the narrow tropical 
parts of Mexico and Central America. The retreat of cold at the ter- 
“Among trees fossil remains of magnolia, sassafras, and liqnidamber have been 
found in Greenland, 
