n
the case of leaf-climbers and tendril-bearers, it is this tendency which
has been taken advantage of and increased through natural selection. It
is, however, probable, from reasons which I have assigned in my memoir,
that this will have occurred only with plants which had already acquired
the power of revolving, and had thus become twiners.

I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular revolving
movements, which were at first of no use to them; this movement, as
well as that due to a touch or shake, being the incidental result of
the power of moving, gained for other and beneficial purposes. Whether,
during the gradual development of climbing plants, natural selection
has been aided by the inherited effects of use, I will not pretend to
decide; but we know that certain periodical movements, for instance the
so-called sleep of plants, are governed by habit.

I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural
selection is incompetent to account for the incipient stages of
useful structures; and I have shown, as I hope, that there is no great
difficulty on this head. A good opportunity has thus been afforded for
enlarging a little on gradations of structure, often associated with
strange functions--an important subject, which was not treated at
sufficient length in the former editions of this work. I will now
briefly recapitulate the foregoing cases.

With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged
use of all the parts, together with inheritance, will have aided in an
important manner in their co-ordination. With the many insects which
imitate various objects, there is no improbability in the belief that
an accidental resemblance to some common object was in each case the
foundation for the work of natural selection, since perfected through
the occasional preservation of slight variations which made the
resemblance at all closer; and this will have been carried on as long
as the insect continued to vary, and as long as a more and more perfect
resemblanc