Mimulus offers another instructive case, showing
that the benefit of a cross depends on the previous treatment of the
progenitors: plants which had been self-fertilised for the eight
previous generations were crossed with plants which had been
intercrossed for the same number of generations, all having been kept
under the same conditions as far as possible; seedlings from this cross
were grown in competition with others derived from the same
self-fertilised mother-plant crossed by a fresh stock; and the latter
seedlings were to the former in height as 100 to 52, and in fertility as
100 to 4. An exactly parallel experiment was tried on Dianthus, with
this difference, that the plants had been self-fertilised only for the
three previous generations, and the result was similar though not so
strongly marked. The foregoing two cases of the offspring of Ipomoea and
Eschscholtzia, derived from a cross with a fresh stock, being as much
superior to the intercrossed plants of the old stock, as these latter
were to the self-fertilised offspring, strongly supports the same
conclusion. A cross with a fresh stock or with another variety seems to
be always highly beneficial, whether or not the mother-plants have been
intercrossed or self-fertilised for several previous generations. The
fact that a cross between two flowers on the same plant does no good or
very little good, is likewise a strong corroboration of our conclusion;
for the sexual elements in the flowers on the same plant can rarely have
been differentiated, though this is possible, as flower-buds are in one
sense distinct individuals, sometimes varying and differing from one
another in structure or constitution.
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