Plants of
Ipomoea were intercrossed for nine successive generations; they were
then again intercrossed, and at the same 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.
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