When two varieties which present well-marked differences are crossed,
their descendants in the later generations differ greatly from one
another in external characters; and this is due to the augmentation or
obliteration of some of these characters, and to the reappearance of
former ones through reversion; and so it will be, as we may feel almost
sure, with any slight differences in the constitution of their sexual
elements. Anyhow, my experiments indicate that crossing plants which
have been long subjected to almost though not quite the same conditions,
is the most powerful of all the means for retaining some degree of
differentiation in the sexual elements, as shown by the superiority in
the later generations of the intercrossed over the self-fertilised
seedlings. Nevertheless, the continued intercrossing of plants thus
treated does tend to obliterate such differentiation, as may be inferred
from the lessened benefit derived from intercrossing such plants, in
comparison with that from a cross with a fresh stock. It seems probable,
as I may add, that seeds have acquired their endless curious adaptations
for wide dissemination, not only that the seedlings would thus be
enabled to find new and fitting homes, but that the individuals which
have been long subjected to the same conditions should occasionally
intercross with a fresh stock. (12/11. See Professor Hildebrand's
excellent treatise 'Verbreitungsmittel der Pflanzen' 1873.
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