of the range 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 physica