The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom | Page 8

habeat, quod hujuscemodi flores
nunquam proprio suo pulvere, sed semper eo aliarum suae speciei
impregnentur, merito quaeritur? Certe natura nil facit frustra." Herbert
'Amaryllidaceae, with a Treatise on Cross-bred Vegetables' 1837.) But
none of these distinguished observers appear to have been sufficiently
impressed with the truth and generality of the law, so as to insist on it
and impress their beliefs on others.
In 1862 I summed up my observations on Orchids by saying that nature
"abhors perpetual self-fertilisation." If the word perpetual had been
omitted, the aphorism would have been false. As it stands, I believe
that it is true, though perhaps rather too strongly expressed; and I
should have added the self-evident proposition that the propagation of
the species, whether by self-fertilisation or by cross-fertilisation, or
asexually by buds, stolons, etc. is of paramount importance. Hermann
Muller has done excellent service by insisting repeatedly on this latter
point.
It often occurred to me that it would be advisable to try whether
seedlings from cross-fertilised flowers were in any way superior to
those from self-fertilised flowers. But as no instance was known with
animals of any evil appearing in a single generation from the closest
possible interbreeding, that is between brothers and sisters, I thought
that the same rule would hold good with plants; and that it would be
necessary at the sacrifice of too much time to self-fertilise and
intercross plants during several successive generations, in order to
arrive at any result. I ought to have reflected that such elaborate
provisions favouring cross-fertilisation, as we see in innumerable plants,
would not have been acquired for the sake of gaining a distant and
slight advantage, or of avoiding a distant and slight evil. Moreover, the
fertilisation of a flower by its own pollen corresponds to a closer form
of interbreeding than is possible with ordinary bi-sexual animals; so

that an earlier result might have been expected.
I was at last led to make the experiments recorded in the present
volume from the following circumstance. For the sake of determining
certain points with respect to inheritance, and without any thought of
the effects of close interbreeding, I raised close together two large beds
of self-fertilised and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully grown were
plainly taller and more vigorous than the self-fertilised ones. Bees
incessantly visit the flowers of this Linaria and carry pollen from one to
the other; and if insects are excluded, the flowers produce extremely
few seeds; so that the wild plants from which my seedlings were raised
must have been intercrossed during all previous generations. It seemed
therefore quite incredible that the difference between the two beds of
seedlings could have been due to a single act of self-fertilisation; and I
attributed the result to the self-fertilised seeds not having been well
ripened, improbable as it was that all should have been in this state, or
to some other accidental and inexplicable cause. During the next year, I
raised for the same purpose as before two large beds close together of
self-fertilised and crossed seedlings from the carnation, Dianthus
caryophyllus. This plant, like the Linaria, is almost sterile if insects are
excluded; and we may draw the same inference as before, 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