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maintained by having some advantage over those with which it comes into
and the consequent extinction of
forms almost inevitably follows. It is the same with our domestic
when a new and slightly improved variety has been raised, it at first
the less improved varieties in the same neighbourhood; when much improved it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. Thus the appearance of new forms and the disappearance of old forms, both
→those naturally and those artificially produced,
are bound together. In
flourishing groups, the number of new specific forms which have been produced within a given time
→has at some periods probably been
greater than
of the old
which have been exterminated; but we know that
→species have
not gone on indefinitely increasing, at least during the later geological
so
looking to later
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
seized on the place occupied by a species belonging to a distinct group, and thus
its
allied forms be developed from the
many will have to yield their places; and it will generally be
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
their places to other
→OMIT
modified and
few of the
may often
→be preserved for a long time,
from being fitted to some peculiar line of life, or from inhabiting some distant and isolated station, where they
escaped severe competition. For instance,
species of Trigonia, a great genus of shells in the secondary formations,
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.
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