As we have already
explained, natural selection is by no means inactive during these
intervening periods of warmth. We have seen the ammonites and
reptiles, and even the birds and mammals, evolve into hundreds of
species during the Jurassic period. The constant evolution of
more effective types of carnivores and their spread into new
regions, the continuous changes in the distribution of land and
water, the struggle for food in a growing population, and a dozen
other causes, are ever at work. But the great and comprehensive
changes in the face of the earth which close the eras of the
geologist seem to give a deeper and quicker stimulus to its
population and result in periods of especially rapid evolution.
Such a change now closes the Mesozoic Era, and inaugurates the
age of flowering plants, of birds, and of mammals.
CHAPTER XIV. IN THE DAYS OF THE CHALK
In accordance with the view of the later story of the earth which
was expressed on an earlier page, we now come to the second of
the three great revolutions which have quickened the pulse of
life on the earth. Many men of science resent the use of the word
revolution, and it is not without some danger.
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