60 The Selection Theory 
As in the case of mimicry many species have come to resemble 
one another through processes of selection, so we know whole classes 
of phenomena in which plants and animals have become adapted to 
one another, and have thus been modified to a considerable degree. 
I refer particularly to the relation between flowers and insects; 
but as there is an article on “The Biology of Flowers” in this 
volume, I need not discuss the subject, but will confine myself 
to pointing out the significance of these remarkable cases for the 
theory of selection. Darwin has shown that the originally incon- 
spicuous blossoms of the phanerogams were transformed into flowers 
through the visits of insects, and that, conversely, several large orders 
of insects have been gradually modified by their association with 
flowers, especially as regards the parts of their body actively concerned. 
Bees and butterflies in particular have become what they are through 
their relation 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 a8 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 trans- 
formed 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, 
