This gradual increase in number of the
species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of
modification and the production of a number of allied forms must be
slow and gradual,--one species giving rise first to two or three
varieties, these being slowly converted into species, which in their
turn produce by equally slow steps other species, and so on, like the
branching of a great tree from a single stem, till the group becomes
large.
ON EXTINCTION.
We have as yet spoken only 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 at successive periods
by catastrophes, 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. Both single species and whole groups of species last for very
unequal periods; some groups, as we have seen, having endured from the
earliest known dawn of life to the present day; some having
disappeared before the close of the palaeozoic period.
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