sterous 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 this 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 coadaptation. At the commencement 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.

From these considerations, I shall devote the first chapter of this
abstract to variation under domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what
is equally or more important, we shall see how great is the power of man
in accumulating by his selection successive slight variations. I will
then pass on to the variability of species in a state of nature; but
I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues of
facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the struggle
for existence among all organic beings throughout the world, which
inevitably follows from the high geometrical ratio of their increase,
will be considered. This is the doctrine of Malthus, applied to the
whole animal and vegetable kingdoms. As many more individuals of each
species are born than can possibly survive; and as, consequently, there
is a frequently recurring struggle for existence, it follows