rmitting a fresh and varied expansion of life, in preparation for the
next great annihilation of the less fit and selection of the more fit.
Life pauses before another leap. The Mesozoic earth--to apply to it the
phrase which a geologist has given to its opening phase--welcomes the
coming and speeds the parting guest. In the depths of the ocean a new
movement is preparing, but we have yet to study the highest forms of
Mesozoic life before we come to the Cretaceous disturbances.

CHAPTER XII. THE AGE OF REPTILES

From one point of view the advance of life on the earth seems to proceed
not with the even flow of a river, but in the successive waves of an
oncoming tide. It is true that we have detected a continuous advance
behind all these rising and receding waves, yet their occurrence is a
fact of some interest, and not a little speculation has been expended on
it. When the great procession of life first emerges out of the darkness
of Archaean times, it deploys into a spreading world of strange
Crustaceans, and we have the Age of Trilobites. Later there is the
Age of Fishes, then of Cryptogams and Amphibia, and then of Cycads and
Reptiles, and there will afterwards be an Age of Birds and Mammals, and
finally an Age of Man. But there is no ground for mystic speculation on
this circumstance of a group of organisms fording the earth for a few
million years, and then perishing or dwindling into insignificance. We
shall see that a very plain and substantial process put an end to the
Age of the Cycads, Ammonites, and Reptiles, and we have seen how the
earlier dynasties ended.

The phrase, however, the Age of Reptiles, is a fitting and true
description of the greater part of the Mesozoic Era, which lies, like
a fertile valley, between the Permian and the Chalk upheavals. From the
bleak heights of the Permian period, or--more probably--from its more
sheltered regions, in which they have lingered with the ferns and
cycads, the reptiles spread out over the earth, as the summer of the
Triassic period advances. In the full warmth and luxuriance of the
Jurassic they become the most singular and powerful army that ever trod
the earth. They include small lizard-like creatures and monsters more
than a hundred feet in length. They swim like whales in the shallow
seas; they shrink into the shell of the giant turtle; they rear
themselves on towering hind limbs, like colossal kangaroos; they even
rise into the air, and fill it with the