s
have in fact separated sexes, and must habitually be crossed. How strange
are these facts! How strange that the pollen and stigmatic surface of the
same flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind, and
some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens, but by those
of the many other flowers on the same plant. How, then, comes it that such
a vast number of the seedlings are mongrelized? I suspect that it must
arise from the pollen of a distinct _variety_ having a prepotent effect
over a 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 directly the
reverse, 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 gigantic tree covered with, {100} 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 that 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, we can see that 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. Asa Gray those