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.
The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects.
Hence the improved and modified descendants of a species will generally
cause the extermination of the parent-species; and if many new forms
have been developed from any one species, the nearest allies of that
species, _i.e._ the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a
new species belonging to some one group will have seized on the place
occupied by a species belonging to a distinct group, and thus caused
its extermination; and if many allied forms be developed from the
successful intruder, many will have to yield their places; and it will
generally be allied forms, which will suffer from some inherited
inferiority in common. But whether it be species belonging to the same
or to a distinct class, which yield their places to other species which
have been modified and improved, a few of the sufferers may often long
be preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a great
genus of shells in the secondary formations, survives in the Australian
seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. 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 pal ozoic 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
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