ng
periods of subsidence. These periods of subsidence would be separated
from each other by immense intervals of time, during which the area
would be either stationary or rising; whilst rising, the fossiliferous
formations on the steeper shores would be destroyed, almost as soon as
accumulated, by the incessant coast-action, as we now see on the shores
of South America. Even throughout the extensive and shallow seas within
the archipelago, sedimentary beds could hardly be accumulated of
great thickness during the periods of elevation, or become capped
and protected by subsequent deposits, so as to have a good chance of
enduring to a very distant future. 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. It is also probable that each great
period of subsidence would be interrupted by oscillations of level,
and that slight climatical changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would
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 plainly leads to the
belief that it would be chiefly these far-ranging species, though
only some of them, which would oftenest produce new varieties; and
the varieties would at first 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. When
such varieties returned to their ancient homes, as they would differ
from their former state in a nearly uniform, though perhaps extremely
slight degree, and as they would be found embedded in slightly
different sub-stages of the same formation, they woul