Clearly
there has been a great selective process analogous to, if not
equal to, the winnowing process at the end of the Palaeozoic. As
there has been a similar, if less considerable, upheaval of the
land, we are at once tempted to think that the great selective
agency was a lowering of the temperature. When we further find
that the most important change in the animal world is the
destruction of the cold-blooded reptiles, which have no concern
for the young, and the luxuriant spread of the warm-blooded
animals, which do care for their young, the idea is greatly
confirmed. When we add that the powerful Molluscs which are
slain, while the humbler Molluscs survive, are those which--to
judge from the nautilus and octopus--love warm seas, the
impression is further confirmed. And when we finally reflect that
the most distinctive phenomenon of the period is the rapid spread
of deciduous trees, it would seem that there is only one possible
interpretation of the Cretaceous Revolution.
This interpretation--that cold was the selecting agency --is a
familiar idea in geological literature, but, as I said, there are
recent writers who profess reserve in regard to it, and it is
proper to glance at, or at least look for, the alternatives.
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