This may occur either at the on-coming of a hot, dry
season or of a cold season (in which the roots absorb less).
Everything suggests that the deciduous tree evolved to meet an
increase of cold, not of heat.
Another suggestion is that animals and plants were not
"climatically differentiated "until the Cretaceous period; that
is to say, that they were adapted to all climates before that
time, and then began to be sensitive to differences of climate,
and live in different latitudes. But how and why they should
suddenly become differentiated in this way is so mysterious that
one prefers to think that, as the animal remains also suggest,
there were no appreciable zones of climate until the Cretaceous.
The magnolia, for instance, flourished in Greenland in the early
Tertiary, and has to live very far south of it to-day. It is much
simpler to assume that Greenland changed--as a vast amount of
evidence indicates--than that the magnolia changed.
Finally, to explain the disappearance of the Mesozoic reptiles
without a fall in temperature, it is suggested that they were
exterminated by the advancing mammals. It is assumed that the
spreading world of the Angiospermous plants somewhere met the
spread of the advancing mammals, and opened out a rich new
granary to them.
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