, should be in so many cases 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 of the
seedlings thus raised turn out, as I found, 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; and the pollen of
each flower readily gets on its stigma without insect agency; for I
have found that plants carefully protected from insects produce the
full number of pods. 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. Asa Gray those
of the United States, and the result was as I anticipated. On the
other hand, Dr. Hooker informs me that the rule does not hold good in
A