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 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 they will partake of some
inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must
marvel, let it be at our presumption in imagining for a mome