tinction 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. {321} 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 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 {322} 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 the