656 THE ANCIENT LAKES OF WESTERN AMERICA: 
gradual (for nature does not often "turn a corner"), but it is 
plain that it must have resulted in the gradual driving south- 
ward of all the varied forms of animal and vegetable life 
that were spread over the continent to the Arctic Sea. 
When glaciers reached as far south as the fortieth parallel it 
is evident that a cold-temperate climate prevailed in Mexico, 
and that only in the south of Mexico would the average an- 
nual temperature have been what it was previously in the 
latitude of New York. We must conclude, therefore, that 
the herds of mammals which once covered the plains of the 
interior of North America were forced by the advancing cold 
into such narrow limits in Southern Mexico that nearly all 
were exterminated. Plants bore their expatriation better; 
inasmueh as a tree, even of the most gigantic size, will live 
upon the space occupied by its roots provided the climatic. 
conditions are favorable; while one of the larger mammals 
would require at least a thousand times this space for its 
support. As a consequence, we find the present flora of our 
continent much more like that of the Miocene than is our 
fauna, though the change to which I have referred seems 
to have been fatal to quite a number of the most abundant 
and interesting of our Miocene forest trees. Of these, the 
Glyptostrobus may be taken as an example. This was 4 
beautiful conifer which, in Miocene times, grew all over our 
continent and over Northern Europe. In the change to the 
glacial period, however, it was exterminated, both there and 
here, yet continued to exist in China—where a Miocene col- 
ony from America had taken root—and it is growing there 
at the present time. This great ice-wedge iih came down 
from the north separated very widely many elements in our 
Miocene flora which have never since been re-united, so that 
when the storm had passed and better days had come, and 
the Mississippi Valley and the Atlantic States were re-pos- 
sessed by the descendants of the Tertiary plants, they were 
still separated, by many thousand miles, from their brethren 
which had formerly crossed the now submerged bridge of 
