But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly modified;
the mutual relations of many of the other inhabitants being thus
disturbed. Nothing can be effected, unless favourable variations
occur, and variation itself is apparently always a very slow process.
The process will often be greatly retarded by free intercrossing. Many
will exclaim that these several causes are amply sufficient wholly to
stop the action of natural selection. I do not believe so. On the
other hand, I do believe that natural selection will always act very
slowly, often only at long intervals of time, and generally on only a
very few of the inhabitants of the same region at the same time. I
further believe, that this very slow, intermittent action of natural
selection accords perfectly well with what geology tells us of the
rate and manner at which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the
amount of change, to the beauty and infinite complexity of the
coadaptations between all organic beings, one with another and with
their physical conditions of life, which may be effected in the long
course of time by nature's power of selection.
EXTINCTION.
This subject will be more fully discussed in our chapter on Geology;
but it must be here alluded to from being intimately connected with
natural selection.
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