the onward advance of life than to study organs in detail--a vast
subject--or construct pedigrees. We therefore pass on to consider the
next great stride that is taken by the advancing life of the earth.
Millions of years of genial climate and rich vegetation have filled
the earth with a prolific and enormously varied population. Over this
population the hand of natural selection is outstretched, as it were,
and we are about to witness another gigantic removal of older types of
life and promotion of those which contain the germs of further advance.
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. It was once thought that the earth was really
shaken at times by vast and sudden cataclysms, which destroyed its
entire living population, so that new kingdoms of plants and animals
had to be created. But we have interpreted the word revolution in a very
different sense. The series of changes and disturbances to which we give
the name extended over a period of hundreds of thousands of years,
and they were themselves, in some sense, the creators of new types
of organisms. Yet they are periods that stand out peculiarly in
the comparatively even chronicle of the earth. The Permian period
transformed the face of the earth; it lifted the low-lying land into
a massive relief, drew mantles of ice over millions of miles of its