ed varieties
will be enabled to spread: there will be much extinction of the less
improved forms, and the relative proportional numbers of the various
inhabitants of the renewed continent will again be changed; and again there
will be a fair field for natural selection to improve still further the
inhabitants, and thus produce new species.

That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of nature,
which can be better occupied by some of the inhabitants of the country
undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and on the
immigration of better adapted forms having been checked. 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 always acts 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, {109} 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. Natural selection acts solely through the
preservation of variations in some way advantageous, which consequently
endure. But as from the high geometrical ratio of increase of all organic
beings,