Chapter 12

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation injurious.
Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation
avoided.
The benefits and evils of the two processes depend on the degree of differentiation in the sexual elements.
The evil effects not due to the combination of morbid tendencies in the parents.
Nature of the conditions to which plants are subjected when growing near together in a state of nature
or under culture, and the effects of such conditions.
Theoretical considerations with respect to the interaction of differentiated sexual elements.
Practical lessons.
Genesis of the two sexes.
Close correspondence between the effects of cross-fertilisation and self-fertilisation, and of the legitimate and illegitimate unions of heterostyled plants, in comparison with hybrid unions.

The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations. So it must be, owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together. The same inference may be drawn still more surely with respect to those plants, such as Reseda and Eschscholtzia, which are sterile with their own pollen, but fertile with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in my experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation.

With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomoea were intercrossed for nine successive generations; they were then again intercrossed, and at the same