in the cold nights to keep warm enough;
the slow, the weak, and the thinly clad soon die off. Again and again,
in each succeeding generation, the same thing takes place. By this
natural process, which is so inevitable that it cannot be conceived not
to act, those best adapted to live, live; those least adapted, die. It
is sometimes said that we have no direct evidence of the action of this
selecting power in nature. But it seems to me we have better evidence
than even direct observation would be, because it is more universal,
viz., the evidence of necessity. It must be so; for, as all wild animals
increase in a geometrical ratio, while their actual numbers remain on
the average stationary, it follows, that as many die annually as are
born. If, therefore, we deny natural selection, it can only be by
asserting that, in such a case as I have supposed, the strong, the
healthy, the swift, the well clad, the well organised animals in every
respect, have no advantage over,--do not on the average live longer
than, the weak, the unhealthy, the slow, the ill-clad, and the
imperfectly organised individuals; and this no sane man has yet been
found hardy enough to assert. But this is not all; for the offspring on
the average resemble their parents, and the selected portion of each
succeeding generation will therefore be stronger, swifter, and more
thickly furred than the last; and if this process goes on for thousands
of generations, our animal will have again become thoroughly in harmony
with the new conditions in which it is placed. But it will now be a
different creature. It will be not only swifter and stronger, and more
furry, it will also probably have changed in colour, in form, perhaps
have acquired a longer tail, or differently shaped ears; for it is an
ascertained fact, that when one part of an animal is modified, some
other parts almost always change, as it were in sympathy with it. Mr.
Darwin calls this "correlation of growth," and gives as instances, that
hairless dogs have imperfect teeth; white cats, when blue-eyed, are
deaf; small feet accompany short beaks in pigeons; and other equally
interesting cases.

Grant, therefore, the premises: 1st. That peculiarities of every kind
are more or less hereditary. 2nd. That the offspring of every animal
vary more or less in all parts of their organization. 3rd. That
the universe in which these animals live, is not absolutely
invariable;--none of which propositions can be denied