Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

36 ON THE TENDENCY OF VARIETIES TO DEPARTl
irruption of some new carnivorous animal seeking
4 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. The 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 coulMd 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 re