CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
4. ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION. (continued)
Formations rich in fossils of many kinds, and of thickness sufficient to
last to an age as distant in futurity as the secondary formations lie in
the past, would generally be formed in the archipelago only during 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
would, according to the principles followed by many palaeontologists, be
ranked as new and distinct species.