6. Dianthus caryophyllus.
Plants self-fertilised for three generations were crossed with pollen
from a fresh stock, and their offspring were grown in competition with
plants of the fourth self-fertilised generation. The crossed plants thus
obtained were to the self-fertilised in height as 100 to 81, and in
fertility (both lots being left to be naturally fertilised by insects)
as 100 to 33.
These same crossed plants were also to the offspring from the plants of
the third generation crossed by the intercrossed plants of the
corresponding generation, in height as 100 to 85, and in fertility as
100 to 45.
We thus see what a great advantage the offspring from a cross with a
fresh stock had, not only over the self-fertilised plants of the fourth
generation, but over the offspring from the self-fertilised plants of
the third generation, when crossed by the intercrossed plants of the old
stock.
7. Pisum sativum.
It has been shown under the head of this species, that the several
varieties in this country almost invariably fertilise themselves, owing
to insects rarely visiting the flowers; and as the plants have been long
cultivated under nearly similar conditions, we can understand why a
cross between two individuals of the same variety does not do the least
good to the offspring either in height or fertility. This case is almost
exactly parallel with that of Mimulus, or that of the Ipomoea named
Hero; for in these two instances, crossing plants which had been
self-fertilised for seven generations did not at all benefit the
offspring.
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