t,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants. Some holly-trees bear only male flowers, which have four stamens
producing a rather 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 a few pollen-grains, and on some a profusion. 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, which had 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 might be effected. It would take up
too much space to show the various steps, through dimorphism and other
means, by which the separation of the sexes in plants of various kinds
is apparently now in progress; but I may add that some of the species
of holly in North America are, according to Asa Gray, in