") But we must not allow this highly
generalised view, or the analogy of chemical affinity, to conceal from
us our ignorance. We do not know what is the nature or degree of the
differentiation in the sexual elements which is favourable for union,
and what is injurious for union, as in the case of distinct species. We
cannot say why the individuals of certain species profit greatly, and
others very little by being crossed. There are some few species which
have been self-fertilised for a vast number of generations, and yet are
vigorous enough to compete successfully with a host of surrounding
plants. We can form no conception why the advantage from a cross is
sometimes directed exclusively to the vegetative system, and sometimes
to the reproductive system, but commonly to both. It is equally
inconceivable why some individuals of the same species should be
sterile, whilst others are fully fertile with their own pollen; why a
change of climate should either lessen or increase the sterility of
self-sterile species; and why the individuals of some species should be
even more fertile with pollen from a distinct species than with their
own pollen. And so it is with many other facts, which are so obscure
that we stand in awe before the mystery of life.
Under a practical point of view, agriculturists and horticulturists may
learn something from the conclusions at which we have arrived.
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