continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very 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
misseltoe, 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.

The author of the 'Vestiges of Creation' would, I presume, say that,
after a certain unknown number of generations, some bird had given
birth to a woodpecker, and some plant to the misseltoe, and that these
had been produced perfect as we now see them; but this assumption
seems to me to be no explanation, for it leaves the case of the
coadaptations of organic beings to each other and to their physical
conditions of life, untouched and unexplained.

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