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.
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