While the
area existed as a continent the inhabitants will have been numerous in
individuals and kinds, and will have been subjected to severe competition.
When converted by subsidence into large separate islands there will still
have existed many individuals of the same species on each island:
intercrossing on the confines of the range of each new species will have
been checked: after physical changes of any kind immigration will have
been prevented, so that new places in the polity of each island will have
had to be filled up by the modification of the old inhabitants; and time
will have been allowed for the varieties in each to become well modified
and perfected. When, by renewed elevation, the islands were reconverted
into a continental area, there will again have been very severe
competition; the most favoured or improved varieties will have been enabled
to spread; there will have been much extinction of the less improved forms,
and the relative proportional numbers of the various inhabitants of the
reunited continent will again have been changed; and again there will have
been a fair field for natural selection to improve still further the
inhabitants, and thus to produce new species.
That natural selection generally act with extreme slowness I fully admit.
It can act only when there are places in the natural polity of a district
which can be better occupied by the modification of some of its existing
inhabitants.
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