THE EVOLUTION OF MAN 83 
This antiquity vastly exceeds that of the nations of Europe or of the 
Americans or of Africa. Fourth, central Asia is the source of almost 
all of our domestic animals, many of which have been subjected to 
human will and control for thousands of years, and this is equally true 
of many of our domestic plants. This is not due to the fact that man 
first reached civilization in Asia, but rather that he chose for his com- 
panions the highest and best of their several evolutionary lines, and 
Asia was the place of all others upon earth where the evolution in 
general of organic life reached its highest development in late Cenozoic 
time (Williston). Fifth, climatic conditions in Asia in the Miocene 
or early Pliocene were such as to compel the descent of the prehuman 
ancestor from the trees, a step which was absolutely essential to 
further human development. 
Impelling cause.—We look for a geologic cause back of this most 
momentous crisis in the evolution of humanity and we find it in conti- 
nental elevation and consequent increasing aridity of climate, espe- 
cially to the northward of the Himalayas. With this increased aridity 
and tempering of tropical heat came the dwindling of the forested 
areas suitable to primate occupancy. Barrell has suggested that this 
diminution left residual forests comparable to the diminishing lakes 
and ponds of the Devonian, which upon final desiccation compelled 
their denizens to become terrestrial or perish. The dwindling of the 
residual forests would have an effect upon the tree-dwellers which may 
be expressed in precisely the same words. Once upon the ground the 
effect upon even a conservative type—and the primates in general, 
where constant conditions prevail, are slow of change—would be the 
rapid acquisition of such adaptations as were necessary to insure sur- 
vival under the new conditions. The other man-like apes had, 
unfortunately for their further evolution, reached a region where 
tropical forests continued to be available and hence have retained their 
arboreal life and with it a stagnation of progress. The result has been, 
at any rate on the part of the three larger forms, a degeneracy from 
the estate of their common ancestry with mankind; the gibbons seem 
to have deteriorated less, while terrestrial man has risen to the summit 
of primate evolution. 
Time.—The time of the descent is not later than early Pliocene 
nor earlier than Miocene time; when the terrestrial ape-man became 
what we would call human was perhaps later, but certainly during the 
Pliocene, which makes the age of man as such measurable in terms of 
hundreds of thousands of years! 
