ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased
in numbers, during that season of the year when the wolf was hardest
pressed for food. Under such circumstances the swiftest and slimmest
wolves have the best chance of surviving, and so be preserved or
selected, provided always that they retained strength to master their
prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that
this would be the result, than that man should be able to improve the
fleetness of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each man trying to
keep the best dogs without any thought of modifying the breed. I may
add that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting