he period of
exploration. But we shall find that a difference of climate, as compared
with earlier ages, was already evident in the middle of the Tertiary
Era, and it is far more noticeable to-day.

We do not know the causes of this climatic evolution--the point will be
considered more closely in connection with the last Ice-Age--but we see
that it throws a flood of light on the evolution of organisms. It is
one of the chief incarnations of natural selection. Changes in the
distribution of land and water and in the nature of the land-surface,
the coming of powerful carnivores, and other agencies which we have
seen, have had their share in the onward impulsion of life, but the most
drastic agency seems to have been the supervention of cold. The higher
types of both animals and plants appear plainly in response to a
lowering of temperature. This is the chief advantage of studying the
story of evolution in strict connection with the geological record. We
shall find that the record will continue to throw light on our path to
the end, but, as we are now about to approach the most important era
of evolution, and as we have now seen so much of the concrete story of
evolution, it will be interesting to examine briefly some other ways of
conceiving that story.

We need not return to the consideration of the leading schools of
evolution, as described in a former chapter. Nothing that we have seen
will enable us to choose between the Lamarckian and the Weismannist
hypothesis; and I doubt if anything we are yet to see will prove
more decisive. The dispute is somewhat academic, and not vital to a
conception of evolution. We shall, for instance, presently follow the
evolution of the horse, and see four of its toes shrink and disappear,
while the fifth toe is enormously strengthened. In the facts themselves
there is nothing whatever to decide whether this evolution took place
on the lines suggested by Weismann, or on the lines suggested by Lamarck
and accepted by Darwin. It will be enough for us merely to establish
the fact that the one-toed horse is an evolved descendant of a primitive
five-toed mammal, through the adaptation of its foot to running on firm
ground, its teeth and neck to feeding on grasses, and so on.

On the other hand, the facts we have already seen seem to justify the
attitude of compromise I adopted in regard to the Mutationist theory. It
would be an advantage in many ways if we could believe that new species
ar