he particular insects which visited them, so as to
favour in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case of
insects visiting flowers for the sake of collecting pollen instead of
nectar; and as pollen is formed for the sole object of fertilisation, its
destruction appears a simple loss to the plant; yet if a little pollen were
carried, at first occasionally and then habitually, by the pollen-devouring
insects from flower to flower, and a cross thus effected, although
nine-tenths of the pollen were destroyed, it might still be a great gain to
the plant; and those individuals which produced more and more pollen, and
had larger and larger anthers, would be selected. {93}

When our plant, by this process of the continued preservation or natural
selection of more and more attractive flowers, had been rendered highly
attractive to insects, they would, unintentionally on their part, regularly
carry pollen from flower to flower; and that they 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 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 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 "phy