of each species will thus be checked: after physical changes of any
kind, immigration will be prevented,
so that new places in the polity of each island will have to be filled
up by modifications of the old inhabitants; and time will be allowed
for the varieties in each to become well modified and perfected. When,
by renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved 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 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
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