On a small island, the race for life will have been less
severe, and there will have been less modification and less
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, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely.
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