siological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one flower or
on one whole plant, and pistils alone in {94} another flower or on another
plant. In plants under culture and placed under new conditions of life,
sometimes the male organs and sometimes the female organs become more or
less impotent; now if we suppose this to occur in ever so slight a degree
under nature, then as pollen is already carried regularly from flower to
flower, and as a more complete separation of the sexes of our plant would
be advantageous on the principle of the division of labour, individuals
with this tendency more and more increased, would be continually favoured
or selected, until at last a complete separation of the sexes would be
effected.

Let us now turn to the nectar-feeding insects in our imaginary case: we may
suppose the plant of which we have been slowly increasing the nectar by
continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many facts,
showing how anxious bees are to save time; for instance, their habit of
cutting holes and sucking the nectar at the bases of certain flowers, which
they can, with a very little more trouble, enter by the mouth. Bearing such
facts in mind, I can see no reason to doubt that an accidental deviation in
the size and form of the body, or in the curvature and length of the
proboscis, &c., far too slight to be appreciated by us, might profit a bee
or other insect, so that an individual so characterised would be able to
obtain its food more quickly, and so have a better chance of living and
leaving descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of the
common red and incarnate clovers (Trifolium pratense and incarnatum) do not
on a hasty glance appear to differ in length; yet the hive-bee can easily
suck the nectar out of the incarnate clover, but not out of the common red
{95} clover, which is visited by humble-bees alone; so that whole fields of
the red clover offer in vain an abundant supply of precious nectar to the
hive-bee. Thus it might be a great advantage to the hive-bee to have a
slightly longer or differently constructed proboscis. On the other hand, I
have found by experiment that the fertility of clover depends on bees
visiting and moving parts of the corolla, so