How, then, comes it that such a vast number of the seedlings are
mongrelized? It must arise from the pollen of a distinct VARIETY having a
prepotent effect over the flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct
individuals of the same species. When distinct SPECIES are crossed the
case is reversed, for a plant's own pollen is always prepotent over foreign
pollen; but to this subject we shall return in a future chapter.
In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at most
only from flower to flower on the same tree; and flowers on the same tree
can be considered as distinct individuals only in a limited sense. I
believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, pollen must be regularly
carried from flower to flower; and this will give a better chance of pollen
being occasionally carried from tree to tree. That trees belonging to all
orders have their sexes more often separated than other plants, I find to
be the case in this country; and at my request Dr. Hooker tabulated the
trees of New Zealand, and Dr.
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