less improved forms,
and the relative proportional numbers of the various inhabitants of the
reunited continent will again have been changed; and again there will
have been a fair field for natural selection to improve still further
the inhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully
admit. It can act only when there are places in the natural polity of a
district which can be better occupied by the modification of some of its
existing inhabitants. The occurrence of such places will often depend
on physical changes, which generally take place very slowly, and on the
immigration of better adapted forms being prevented. As some few of
the old inhabitants become modified the mutual relations of others will
often be disturbed; and this will create new places, ready to be filled
up by better adapted forms; but all this will take place very slowly.
Although all the individuals of the same species differ in some slight
degree from each other, it would often be long before differences of
the right nature in various parts of the organisation might occur. The
result would often be greatly retarded by free intercrossing. Many will
exclaim that these several causes are amply sufficient to neutralise the
power of natural selection. I do not believe so. But I do believe that
natural selection will generally act very slowly, only at long intervals
of time, and only on a few of the inhabitants of the same region. I
further believe that these slow, intermittent results accord well with
what geology tells us of the rate and manner at which the inhabitants of
the world have changed.

Slow though the process of selection may be, if feeble man can do much
by artificial selection, I can see no limit to the amount of change,
to the beauty and complexity of the coadaptations between all organic
beings, one with another and with their physical conditions of life,
which may have been effected in the long course of time through nature's
power of selection, that is by the survival of the fittest.

EXTINCTION CAUSED BY NATURAL SELECTION.

This subject will be more fully discussed in our chapter on Geology; but
it must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, e