ated, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified so as to acquire that perfection of structure and
co-adaptation which justly excites our admiration.

Naturalists continually refer to external conditions, such as climate,
food, etc., as the only possible cause of variation. In one limited
sense, as we shall hereafter see, this may be true; but it is
preposterous to attribute to mere external conditions the structure, for
instance, of the woodpecker, with its feet, tail, beak, and tongue, so
admirably adapted to catch insects under the bark of trees. In the case
of the mistletoe, which draws its nourishment from certain trees, which
has seeds that must be transported by certain birds, and which has
flowers with separate sexes absolutely requiring the agency of certain
insects to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of the parasite, with its
relations to several distinct organic beings, by the effects of external
conditions, or of habit, or of the volition of the plant itself.

It is, therefore, of the highest importance to gain a clear insight into
the means of modification and co-adaptation. At the beginning of my
observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been disappointed;
in this and in all other perplexing cases I have invariably found that
our knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my
conviction of the high value of such studies, although they have been
very commonly neglected by naturalists.

Although much remains obscure, and will long remain obscure, I can
entertain no doubt, after the most deliberate study and dispassionate
judgment of which I am capable, that the view which most naturalists
until recently entertained, and which I formerly entertained--namely,
that each species has been independently created--is erroneous. I am
fully convinced that species are not immutable, but that those belonging
to what are called the same genera are lineal descendants of some other
and generally extinct species, in the same manner as the acknowledged
v