, which seed much more freely when fertilised
with the pollen of a distinct species than when fertilised with pollen from
the same plant.
We thus see that, although there is a clear and great 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 in their reproductive systems. These differences in
both cases follow, to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to indicate that the greater or lesser difficulty of
either grafting or crossing various species has been a special endowment;
although in the case of crossing, the difficulty is as important for the
endurance and stability of specific forms as in the case of grafting it is
unimportant for their welfare.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly acquired
through the natural selection of slightly lessened degrees of fertility,
which, like any other variation, spontaneously appeared in certain
individuals of one variety when crossed with those of another variety.
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