Now the active quality was that of the species, and its
latent state was found in the variety. Hence the inference that hybrids
between a species and its retrograde variety will bear the aspect of the
species. This attribute may be fully developed, and then the hybrid will
not be distinguishable from the pure species in its outer appearance. Or
the character may be incompletely evolved, owing to the failure of
cooperation of the dormant unit. In this case the hybrid will be in some
sense intermediate between its parents, but these instances are more
rare than the alternate ones, though presumably they may play an
important part in the variability of many hybrid garden-flowers.
All of these three rules are supported by a large amount of evidence.
The complete fertility of varietal hybrids is so universally
acknowledged that it is not worth while to give special instances. With
many prominent systematists it has become a test between species and
varieties, and from our present point of view this assumption is
correct. Only the test is of little use in practice, as fertility may be
diminished [279] in unbalanced unions in all possible degrees, according
to the amount of difference between the parents. If this amount is
slight, if for instance, only one unit-character causes the difference,
the injury to fertility may, be so small as to be practically nothing.
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