During the periods of subsidence, there would probably be much extinction of life; during the periods of elevation, there would be much variation, but the geological record would then be less perfect.

It may be doubted whether the duration of any one great period of subsidence over the whole or part of the archipelago, together with a contemporaneous accumulation of sediment, would EXCEED the average duration of the same specific forms; and these contingencies are indispensable for the preservation of all the transitional gradations between any two or more species.

If such gradations were not all fully preserved, transitional varieties would merely appear as so many new, though closely allied species.