that continental productions have
everywhere become so largely naturalised on islands. On a small island, the
race for life will have been less severe, and there will have been less
modification and less {107} extermination. Hence, perhaps, it comes that
the flora of Madeira, according to Oswald Heer, resembles the extinct
tertiary flora of Europe. All fresh-water basins, taken together, make a
small area compared with that of the sea or of the land; and, consequently,
the competition between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed, and old forms
more slowly exterminated. And it is in fresh water that we find seven
genera of Ganoid fishes, remnants of a once preponderant order: and in
fresh water we find some of the most anomalous forms now known in the
world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect
to a certain extent orders now widely separated in the natural scale. These
anomalous forms may almost be called living fossils; they have endured to
the present day, from having inhabited a confined area, and from having
thus been exposed to less severe competition.

To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a broken condition,
is the most favourable for the production of many new forms of life, likely
to endure long and to spread widely. For the area first 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 {108} 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 improv