A final word may be added regarding the probable climate of the region
during the lifetime of these fossil forests. It is obvious that the
present flora of the Yellowstone National Park has comparatively little
relation to the Tertiary flora and can not be considered the descendant
of it. It is also clear that the climatic conditions must have greatly
changed since Tertiary time. The Tertiary flora appears to have come
from the south, but the present flora is evidently of more northern
origin. The climate during Tertiary time, as indicated by the
vegetation, was temperate or warm-temperate, not unlike that of Virginia
or the Carolinas at the present time, and the presence of numerous
species of figs, a supposed bread-fruit tree, cinnamons, bays, and other
southern plants indicates that it may have been almost subtropical.
However, the conditions that were favorable to this seemingly
subtropical growth may have been different from the conditions now
necessary for the growth of similar vegetation. It may be that these
supposed subtropical plants were at that time so constituted as to grow
in a temperate land, and that they may have become tropical in recent
times. Following this general line of thought it may be said that
although the Tertiary vegetation of the Yellowstone National Park would
now be regarded as indicating a temperate or even warmer climate, the
actual climate may not have been subtropical. It is certain, however,
that the conditions were very different from those now prevailing in the
park.