We see in many
cases in the more recent tertiary formations, that rarity precedes
extinction; and we know that this has been the progress of events with
those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely,
that to admit that species generally become rare before they become
extinct--to feel no surprise at the rarity of a species, and yet to
marvel greatly when it ceases to exist, is much the same as to admit
that sickness in the individual is the forerunner of death--to feel no
surprise at sickness, but when the sick man dies, to wonder and to
suspect that he died by some unknown deed of violence.
The theory of natural selection is grounded on the belief that each
new variety, and ultimately each new species, is produced and
maintained by having some advantage over those with which it comes
into competition; and the consequent extinction of less-favoured forms
almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised,
it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near, like
our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance of
old forms, both natural and artificial, are bound together. In certain
flourishing groups, the number of new specific forms which have been
produced within a given time is probably greater than that of the old
forms which have been exterminated; but we know that the number of
species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may
believe that the production of new forms has caused the extinction of
about the same number of old forms.
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