The
seedlings thus raised were grown in competition with each other in the
usual manner, and differed in their powers of growth. Those from the
self-fertilised plants which had been crossed during the two previous
generations were to those from the plants self-fertilised during many
previous generations in height as 100 to 90. These two lots of seeds
were likewise tried by being sown under very unfavourable conditions in
poor exhausted soil, and the plants whose grandparents and
great-grandparents had been crossed showed in an unmistakable manner
their superior constitutional vigour. In this case, as in that of the
heartsease, there could be no doubt that the advantage derived from a
cross between two plants was not confined to the offspring of the first
generation. That constitutional vigour due to cross-parentage is
transmitted for many generations may also be inferred as highly
probable, from some of Andrew Knight's varieties of the common pea,
which were raised by crossing distinct varieties, after which time they
no doubt fertilised themselves in each succeeding generation. These
varieties lasted for upwards of sixty years, "but their glory is now
departed." (8/3. See the evidence on this head in my 'Variation under
Domestication' chapter 9 volume 1 2nd edition page 397.) On the other
hand, most of the varieties of the common pea, which there is no reason
to suppose owe their origin to a cross, have had a much shorter
existence.
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