Mr. C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid between Rhododendron Ponticum
and Catawbiense, and that this hybrid "seeds as freely as it is
possible to imagine." Had hybrids, when fairly treated, gone on
decreasing in fertility in each successive generation, as Gartner
believes to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrids, and
such alone are fairly treated, for by insect agency the several
individuals of the same hybrid variety are allowed to freely cross
with each other, and the injurious influence of close interbreeding is
thus prevented. Any one may readily convince himself of the efficiency
of insect-agency by examining the flowers of the more sterile kinds of
hybrid rhododendrons, which produce no pollen, for he will find on
their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is if the genera of animals are as distinct from each other, as are
the genera of plants, then we may infer that animals more widely
separated in the scale of nature can be more easily crossed than in
the case of plants; but the hybrids themselves are, I think, more
sterile. I doubt whether any case of a perfectly fertile hybrid animal
can be considered as thoroughly well authenticated.
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