al superiority. Now, let some
alteration of physical conditions occur in the district--a long period
of drought, a destruction of vegetation by locusts, the irruption of
some new carnivorous animal seeking "pastures new"--any change in fact
tending to render existence more difficult to the species in question,
and tasking its utmost powers to avoid complete extermination; it is
evident that, of all the individuals composing the species, those
forming the least numerous and most feebly organized variety would
suffer first, and, were the pressure severe, must soon become extinct.
The same causes continuing in action, the parent species would next
suffer, would gradually diminish in numbers, and with a recurrence of
similar unfavourable conditions might also become extinct. Tho superior
variety would then alone remain, and on a return to favourable
circumstances would rapidly increase in numbers and occupy the place of
the extinct species and variety.

_Superior Varieties will ultimately Extirpate the original Species._

The _variety_ would now have replaced the _species_, of which it would
be a more perfectly developed and more highly organized form. It would
be in all respects better adapted to secure its safety, and to prolong
its individual existence and that of the race. Such a variety _could
not_ return to the original form; for that form is an inferior one, and
could never compete with it for existence. Granted, therefore, a
"tendency" to reproduce the original type of the species, still the
variety must ever remain preponderant in numbers, and under adverse
physical conditions _again alone survive_. But this new, improved, and
populous race might itself, in course of time, give rise to new
varieties, exhibiting several diverging modifications of form, any of
which, tending to increase the facilities for preserving existence,
must, by the same general law, in their turn become predominant. Here,
then, we have _progression and continued divergence_ deduced from the
general laws which regulate the existence of animals in a state of
nature, and from the undisputed fact that varieties do frequently occur.
It is not, however, contended that this result would be invariable; a
change of physical conditions in the district might at times materially
modify it, rendering the race which had been the most capable of
supporting existence under the former conditions now the least so, and
even causing the extinction of the newe