There is another subject on which some light is thrown by the facts
given in this volume, namely, hybridisation. It is notorious that when
distinct species of plants are crossed, they produce with the rarest
exceptions fewer seeds than the normal number. This unproductiveness
varies in different species up to sterility so complete that not even an
empty capsule is formed; and all experimentalists have found that it is
much influenced by the conditions to which the crossed species are
subjected. The pollen of each species is strongly prepotent over that of
any other species, so that if a plant's own pollen is placed on the
stigma some time after foreign pollen has been applied to it, any effect
from the latter is quite obliterated. It is also notorious that not only
the parent species, but the hybrids raised from them are more or less
sterile; and that their pollen is often in a more or less aborted
condition. The degree of sterility of various hybrids does not always
strictly correspond with the degree of difficulty in uniting the parent
forms. When hybrids are capable of breeding inter se, their descendants
are more or less sterile, and they often become still more sterile in
the later generations; but then close interbreeding has hitherto been
practised in all such cases. The more sterile hybrids are sometimes much
dwarfed in stature, and have a feeble constitution.
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