Therefore the
utter extinction of a group is generally, as we have seen, a slower
process than its production.
With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palaeozoic period and
of Ammonites at the close of the secondary period, we must remember
what has been already said on the probable wide intervals of time
between our consecutive formations; and in these intervals there may
have been much slow extermination. Moreover, when by sudden
immigration or by unusually rapid development, many species of a new
group have taken possession of a new area, they will have exterminated
in a correspondingly rapid manner many of the old inhabitants; and the
forms which thus yield their places will commonly be allied, for they
will partake of some inferiority in common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must
marvel, let it be at our presumption in imagining for a moment that we
understand the many complex contingencies, on which the existence of
each species depends. If we forget for an instant, that each species
tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will
be utterly obscured. Whenever we can precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not till
then, we may justly feel surprise why we cannot account for the
extinction of this particular species or group of species.
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