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 least 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 fully
preserved, transitional varieties would merely appear as so many
distinct species. It is, also, probable that each great period of
subsidence would be interrupted by oscillations of level, and that
slight climatal changes would intervene during such lengthy periods;
and in these cases the inhabitants of the archipelago would have to
migrate, and no closely consecutive record of their modifications
could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to
believe that it would be chiefly these far-ranging species which would
oftenest produce new varieties; and the varieties would at first
generally be local or confined to one place, but if possessed of any
decided advantage, or when further modified and improved, they would
slowly spread and supplant their parent-forms.
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