For the area will first have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will
have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many
individuals of the same species on each island: intercrossing on the
confines 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.
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