ose 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.

There remain one or two conceptions of evolution which we have not
hitherto noticed, as it was advisable to see the facts first. One of
these is the view--chiefly represented in this country by Professor
Henslow--that natural selection has had no part in the creation of
species; that the only two factors are the environment and the organism
which responds to its changes. This is true enough in the sense that, as
we saw, natural selection is not an action of nature on the "fit," but
on the unfit or less fit. But this does not in the least lessen the
importance of natural selection. If there were not in nature this body
of destructive agencies, to which we apply the name natural selection,
there would be little--we cannot say no--evolution. But the rising
carnivores, the falls of temperature, etc., that we have studied, have
had so real, if indirect, an influence on the development of life that
we need not dwell on this.

Another school, or several schools, while admitti