Chap. IV. 
TO NATUKAL SELECTION. 
107 
mination. 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 slowdy 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 un¬ 
favourable 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 pre- 
