namely, that the parent-plants must have been intercrossed during every
or almost every previous generation. Nevertheless, the self-fertilised
seedlings were plainly inferior in height and vigour to the crossed.

My attention was now thoroughly aroused, for I could hardly doubt that
the difference between the two beds was due to the one set being the
offspring of crossed, and the other of self-fertilised flowers.
Accordingly I selected almost by hazard two other plants, which happened
to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea
purpurea, both of which, unlike the Linaria and Dianthus, are highly
self-fertile if insects are excluded. Some flowers on a single plant of
both species were fertilised with their own pollen, and others were
crossed with pollen from a distinct individual; both plants being
protected by a net from insects. The crossed and self-fertilised seeds
thus produced were sown on opposite sides of the same pots, and treated
in all respects alike; and the plants when fully grown were measured and
compared. With both species, as in the cases of the Linaria and
Dianthus, the crossed seedlings were conspicuously superior in height
and in other ways to the self-fertilised. I therefore determined to
begin a long series of experiments with various plants, and these were
continued for the following eleven years; and we shall see that in a
large majority of cases the crossed beat the self-fertilised plants.
Several of the exceptional cases, moreover, in which the crossed plants
were not victorious, can be explained.

It should be observed that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds,
seedlings, or plants; these terms implying that they are the product of
crossed or self-fertilised flowers. Cross-fertilisation always means a
cross between distinct plants which were raised from seeds and not from