: "It is very remarkable that,
although there is a great difference in the form of the flower,
especially of the tube, of P. nyctanigenaeflora and P. phoenicea the
mules between them are not only fertile, but I have found them seed much
more freely with me than either parent.... From a pod of the
above-mentioned mule, to which no pollen but its own had access, I had a
large batch of seedlings in which there was no variability or difference
from itself; and it is evident that the mule planted by itself, in a
congenial climate, would reproduce itself as a species; at least as much
deserving to be so considered as the various Calceolarias of different
districts of South America."[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. Yet these few seeds were fertile; so that in this case the
first cross was almost sterile, though the hybrid when at length
produced was fertile. In like manner, dissimilarly coloured varieties of
Verbascum or mullein have been found by two distinct observers to be
comparatively infertile. The two pimpernels (Anagallis arvensis and A.
coerulea), classed by most botanists as varieties of one species, have
been found, after repeated trials, to be perfectly sterile when crossed.

No cases of this kind are recorded among animals; but this is not to be
wondered at, w