ON EXTINCTION.
We have as yet only spokesn incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an island,
the process of extinction may have been rapid. Both single species and
whole groups of species last for very unequal periods; some groups, as we
have seen, have endured from the earliest known dawn of life to the present
day; some have disappeared before the close of the palaeozoic period. No
fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the
extinction of a whole group of species is generally a slower process than
their production: if their appearance and disappearance be represented, as
before, by a vertical line of varying thickness the line is found to taper
more gradually at its upper end, which marks the progress of extermination,
than at its lower end, which marks the first appearance and the early
increase in number of the species.
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