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

I,'DEFIlITELY FROM THE ORIGINAL TYPE. 37
main 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 newer and,
for a time, superior race, while the old or parent
species and its first inferior varieties continued to
flourish. Variations in unimportant parts might
also occur, having no perceptible effect on the lifepreserving powers; and the varieties so furnished
might run a course parallel with the parent species,
either giving rise to further variations or returning
to the former type. All we argue for is, that certain varieties have a tendency to maintain their
existence longer than the original species, and this
tendency must make itself felt; for though the doctrine of chances or averages can never be trusted to