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Climatic Evolution
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 arose by sudden and large variations (mutations)
of the young from the parental type. In the case of many organs
and habits it is extremely difficult to see how a gradual
development, by a slow accentuation of small variations, is
possible. When we further find that experimenters on living
species can bring about such mutations, and when we reflect that
there must have been acute disturbances in the surroundings of
animals and plants sometimes, we are disposed to think that many
a new species may have arisen in this way. On the other hand,
while the palaeontological record can never prove that a species
arose by mutations, it does sometimes show that species arise by
very gradual modification. The Chalk period, which we have just
traversed, affords a very clear instance. One of our chief
investigators of the English Chalk, Dr. Rowe, paid particular
attention to the sea-urchins it contains, as they serve well to
identify different levels of chalk. He discovered, not merely
that they vary from level to level, but that in at least one
genus (Micraster) he could trace the organism very gradually
passing from one species to another, without any leap or
abruptness. It is certainly significant that we find such cases
as this precisely where the conditions of preservation are
exceptionally good. We must conclude that species arise,
probably, both by mutations and small variations, and that it is
impossible to say which class of species has been the more
numerous.
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