from the foregoing facts, that
the varying offspring of each species will try (only few will succeed)
to seize on as many and as diverse places in the economy of nature as
possible. Each new variety or species, when formed, will generally take
the place of, and thus exterminate its less well-fitted parent. This I
believe to be the origin of the classification and affinities of organic
beings at all times; for organic beings always _seem_ to branch and
sub-branch like the limbs of a tree from a common trunk, the flourishing
and diverging twigs destroying the less vigorous--the dead and lost
branches rudely representing extinct genera and families.

This sketch is _most_ imperfect; but in so short a space I cannot make
it better. Your imagination must fill up very wide blanks.

C. DARWIN.

III. _On the Tendency of Varieties to depart indefinitely from the
Original Type._ By ALFRED RUSSELL WALLACE.

One of the strongest arguments which have been adduced to prove the
original and permanent distinctness of species is, that _varieties_
produced in a state of domesticity are more or less unstable, and often
have a tendency, if left to themselves, to return to the normal form of
the parent species; and this instability is considered to be a
distinctive peculiarity of all varieties, even of those occurring among
wild animals in a state of nature, and to constitute a provision for
preserving unchanged the originally created distinct species.

In the absence or scarcity of facts and observations as to _varieties_
occurring among wild animals, this argument has had great weight with
naturalists, and has led to a very general and somewhat prejudiced
belief in the stability of species. Equally general, however, is the
belief in what are called "permanent or true varieties,"--races of
animals which continually propagate their like, but which differ so
slightly (although constantly) from some other race, that the one is
considered to be a _variety_ of the other. Which is the _variety_ and
which the original _species_, there is generally no means of
determining, except in those rare cases in which the one race has been
known to produce an offspring unlike itself and resembling the other.
This, however, would seem quite incompatible with the "permanent
invariability of species," but the difficulty is overcome by assuming
that such varieties have strict limits, and can never again vary further
from the original type, although they