When many
of the inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the all-important
relations of organism to organism in the struggle for life, that any form
which did not become in some degree modified and improved, would be liable
to extermination. Hence, we see why all the species in the same region do
at last, if we look to long enough intervals of time, become modified; for
otherwise they would become extinct.
In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly intermittent
intervals of time; consequently the amount of organic change exhibited by
the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in an ever slowly
changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the place
of another species in the economy of nature, and thus supplant it; yet the
two forms--the old and the new--would not be identically the same; for both
would almost certainly inherit different characters from their distinct
progenitors; and organisms already differing would vary in a different
manner.
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