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

RESEMBLANCES AMONG ANIMALS. 67
temperate into an arctic district, the conditions are
changed. During a large portion of the year, and
just when the strucgle for existence is most severe,
white is the prevailing tint of nature, and dark colours
will be the most conspicuous. The white varieties will
now have an advantage; they will escape from their
enemies or will secure food, while their brown companions will be devoured or will starve; and as " like
produces like" is the established rule in nature, the
white race will become permanently established, and
dark varieties, when they occasionally appear, will soon
die out from their want of adaptation to their environment. In each case the fittest will survive, and a race
will be eventually produced adapted to the conditions
in which it lives.
We have here an illustration of the simple and effectual means by which animals are brought into harmony
with the rest of nature. That slight amount of variability in every species, which we often look upon as
something accidental or abnormal, or so insignificant as
to be hardly worthy of notice, is yet the foundation of
all those wonderful and harmonious resemblances which
play such an important part in the economy of nature.
Variation is generally very small in amount, but it
is all that is required, because the change in the
external conditions to which an animal is subject is
generally very slow and intermittent. When these
changes have taken place too rapidly, the result has
often been the extinction of species; but the general
rule is, that climatal and geological changes go oiu
F2