If, as there is reason to suspect, self-fertilisation
is in some respects beneficial, although more than counterbalanced by
the advantages derived from a cross with a fresh stock, the problem
becomes still more complicated.
As I only twice experimented on more than a single species in a genus, I
cannot say whether the crossed offspring of the several species within
the same genus differ in their degree of superiority over their
self-fertilised brethren; but I should expect that this would often
prove to be the case from what was observed with the two species of
Lobelia and with the individuals of the same species of Nicotiana. The
species belonging to distinct genera in the same family certainly differ
in this respect. The effects of cross- and self-fertilisation may be
confined either to the growth or to the fertility of the offspring, but
generally extends to both qualities. There does not seem to exist any
close correspondence between the degree to which their offspring profit
by this process; but we may easily err on this head, as there are two
means for ensuring cross-fertilisation which are not externally
perceptible, namely, self-sterility and the prepotent fertilising
influence of pollen from another individual. Lastly, it has been shown
in a former chapter that the effect produced by cross and
self-fertilisation on the fertility of the parent-plants does not always
correspond with that produced on the height, vigour, and fertility of
their offspring.
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