"[56]
Darwin was informed by Mr. C. Noble that he raises stocks for grafting
from a hybrid between Rhododendron ponticum and R. catawbiense, and that
this hybrid seeds as freely as it is possible to imagine. He adds that
horticulturists raise large beds of the same hybrid, and such alone are
fairly treated; for, by insect agency, the several individuals are
freely crossed with each other, and the injurious influence of close
interbreeding is thus prevented. Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive generation, as
Gartner believed to be the case, the fact would have been notorious to
nurserymen.[57]
_Cases of Sterility of Mongrels._
The reverse phenomenon to the fertility of hybrids, the sterility of
mongrels or of the crosses between _varieties_ of the same species, is a
comparatively rare one, yet some undoubted cases have occurred. Gartner,
who believed in the absolute distinctness of species and varieties, had
two varieties of maize--one dwarf with yellow seeds, the other taller
with red seeds; yet they never naturally crossed, and, when fertilised
artificially, only a single head produced any seeds, and this one only
five grains.
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