Some also of Mr. Laxton's varieties produced by artificial
crosses have retained their astonishing vigour and luxuriance for a
considerable number of generations; but as Mr. Laxton informs me, his
experience does not extend beyond twelve generations, within which
period he has never perceived any diminution of vigour in his plants.
An allied point may be here noticed. As the force of inheritance is
strong with plants (of which abundant evidence could be given), it is
almost certain that seedlings from the same capsule or from the same
plant would tend to inherit nearly the same constitution; and as the
advantage from a cross depends on the plants which are crossed differing
somewhat in constitution, it may be inferred as probable that under
similar conditions a cross between the nearest relations would not
benefit the offspring so much as one between non-related plants. In
support of this conclusion we have some evidence, as Fritz Muller has
shown by his valuable experiments on hybrid Abutilons, that the union of
brothers and sisters, parents and children, and of other near relations
is highly injurious to the fertility of the offspring. In one case,
moreover, seedlings from such near relations possessed very weak
constitutions. (8/4. 'Jenaische Zeitschrift fur Naturw.' B. 7 pages 22
and 45 1872 and 1873 pages 441-450.) This same observer also found three
plants of a Bignonia growing near together.
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