can most effectually do this, I could easily show by many striking
instances. I will give only one not as a very striking case, but as
likewise illustrating one step in the separation of the sexes of
plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under the
microscope, and on all, without exception, there were pollen-grains,
and on some a profusion of pollen. As the wind had set for several days
from the female to the male tree, the pollen could not thus have been
carried. The weather had been cold and boisterous, and therefore not
favourable to bees, nevertheless every female flower which I examined
had been effectually fertilised by the bees, accidentally dusted with
pollen, having flown from tree to tree in search of nectar. But to
return to our imaginary case: as soon as the plant had been rendered so
highly attractive to insects that pollen was regularly carried from
flower to flower, another process might commence. No naturalist doubts
the advantage of what has been called the physiological 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
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
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