Many of the beautiful pelargoniums of our greenhouses are hybrids, such as P.ignescens from a cross between P.citrinodorum and P.fulgidum, which is quite fertile, and has become the parent of innumerable varieties of beautiful plants.
All the varied species of Calceolaria, however different in appearance, intermix with the greatest readiness, and the hybrids are all more or less fertile.

But the most remarkable case is that of two species of Petunia, of which Dean Herbert says: "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.