Chapter 12 - Page 2

time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An analogous experiment with Eschscholtzia gave a similar result, as far as fertility was concerned. In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was injurious; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly parallel case. With various other plants, the wonderful effects of a cross with a fresh stock may be seen in Table 7/C. Several accounts have also been published of the extraordinary growth of seedlings from a cross between two varieties of the same species, some of which are known never to fertilise themselves; so that here neither self-fertilisation nor relationship even in a remote degree can have come into play. (12/1. See 'Variation under Domestication' chapter 19 2nd edition volume 2 page 159.) We may therefore conclude that the above two propositions are true,--that cross-fertilisation is generally beneficial and self-fertilisation injurious to the offspring.

That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals; but then we must remember that the closest possible interbreeding with such animals, that is, between brothers and sisters, cannot be considered as nearly so close a union as that between the pollen and ovules of the same flower. Whether the evil from self-fertilisation goes on increasing during successive generations is not as yet known; but we may infer from my experiments that the increase if any is far from rapid. After plants have been propagated by self-fertilisation for several generations, a single cross with a fresh stock restores their pristine vigour; and we have a strictly analogous result with our domestic animals. (12/2. Ibid chapter 19 2nd edition volume 2 page 159.) The good effects of cross-fertilisation are transmitted by plants to the next generation; and judging from the varieties of the common pea, to many succeeding generations. But this may merely be that crossed plants of the first generation are extremely vigorous, and transmit their vigour, like any other character, to their successors.

Notwithstanding the evil which many plants suffer from self-fertilisation, they can be thus propagated under favourable conditions for many generations, as shown by some of my