to flowers. In this case again all that is apparently contradictory to the
theory can, on closer investigation, be beautifully interpreted in
corroboration of it. Selection can give rise only to what is of use to the
organism actually concerned, never to what is of use to some other
organism, and we must therefore expect to find that in flowers only
characters of use to THEMSELVES have arisen, never characters which are of
use to insects only, and conversely that in the insects characters useful
to them and not merely to the plants would have originated. For a long
time it seemed as if an exception to this rule existed in the case of the
fertilisation of the yucca blossoms by a little moth, Pronuba yuccasella.
This little moth has a sickle-shaped appendage to its mouth-parts which
occurs in no other Lepidopteron, and which is used for pushing the yellow
pollen into the opening of the pistil, thus fertilising the flower. Thus
it appears as if a new structure, which is useful only to the plant, has
arisen in the insect. But the difficulty is solved as soon as we learn
that the moth lays its eggs in the fruit-buds of the Yucca, and that the
larvae, when they emerge, feed on the developing seeds. In effecting the
fertilisation of the flower the moth is at the same time making provision
for its own offspring, since it is only after fertilisation that the seeds
begin to develop. There is thus nothing to prevent our referring this
structural adaptation in Pronuba yuccasella to processes of selection,
which have gradually transformed the maxillary palps of the female into the
sickle-shaped instrument for collecting the pollen, and which have at the
same time developed in the insect the instinct to press the pollen into the
pistil.

In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any other
explanation, which, now that the facts are so well known, could be regarded
as a serious rival to it. That selection is a factor, and a very powerful