Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty
on another species, when thus grafted were rendered barren. On the
other hand, certain species of Sorbus, when grafted on other species,
yielded twice as much fruit as when on their own roots. We are
reminded by this latter fact of the extraordinary case of Hippeastrum,
Lobelia, etc., which seeded much more freely when fertilised with the
pollen of distinct species, than when self-fertilised with their own
pollen.
We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of crossing
distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of
first crosses, are incidental on unknown differences, chiefly in their
reproductive systems. These differences, in both cases, follow to a
certain extent, as might have been expected, systematic affinity, by
which every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed.
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