changes in the face of the earth and the rise of
its successive dynasties of carnivores have stimulated living things to
higher and higher levels in the primitive ocean. We pass to the clearer
and far more important story of life on land, pursuing the fish through
its continuous adaptations to new conditions until, throwing out
side-branches as it progresses, it reaches the height of bird and mammal
life.

CHAPTER VIII. THE COAL-FOREST

With the beginning of life on land we open a new and more important
volume of the story of life, and we may take the opportunity to make
clearer certain principles or processes of development which we may seem
hitherto to have taken for granted. The evolutionary work is too often
a mere superficial description of the strange and advancing classes of
plants and animals which cross the stage of geology. Why they change and
advance is not explained. I have endeavoured to supply this explanation
by putting the successive populations of the earth in their respective
environments, and showing the continuous and stimulating effect on them
of changes in those environments. We have thus learned to decipher
some lines of the decalogue of living nature. "Thou shalt have a thick
armour," "Thou shalt be speedy," "Thou shalt shelter from the more
powerful," are some of the laws of primeval life. The appearance of each
higher and more destructive type enforces them with more severity; and
in their observance animals branch outward and upward into myriads of
temporary or permanent forms.

But there is no consciousness of law and no idea of evading danger.
There is not even some mysterious instinct "telling" the animal, as
it used to be said, to do certain things. It is, in fact, not strictly
accurate to say that a certain change in the environment stimulates
animals to advance. Generally speaking, it does not act on the advancing
at all, but on the non-advancing, which it exterminates. The procedure
is simple, tangible, and unconscious. Two invading arms of the sea meet
and pour together their different waters and populations. The habits,
the foods, and the enemies of many types of animals are changed; the
less fit for the new environment die first, the more fit survive longest
and breed most of the new generation. It is so with men when they
migrate to a more exacting environment, whether a dangerous trade or
a foreign clime. Again, take the case of the introduction of a giant
Cephalopod or fish