This case is especially valuable, as showing how careful we should be in assuming the infertility of hybrids when experiments have been made with the progeny of a single pair, and have been continued only for one or two generations.
Among insects one case only appears to have been recorded.

The hybrids of two moths (Bombyx cynthia and B.arrindia) were proved in Paris, according to M.Quatrefages, to be fertile _inter se_ for eight generations.
_Fertility of Hybrids among Plants._ Among plants the cases of fertile hybrids are more numerous, owing, in part, to the large scale on which they are grown by gardeners and nurserymen, and to the greater facility with which experiments can be made.

Darwin tells us that Koelreuter found ten cases in which two plants considered by botanists to be distinct species were quite fertile together, and he therefore ranked them all as varieties of each other.
In some cases these were grown for six to ten successive generations, but after a time the fertility decreased, as we saw to be the case in animals, and presumably from the same cause, too close interbreeding.
Dean Herbert, who carried on experiments with great care and skill for many years, found numerous cases of hybrids which were perfectly fertile _inter se_.

Crinum capense, fertilised by three other species--C.
pedunculatum, C.canaliculatum, or C.defixum--all very distinct from it, produced perfectly fertile hybrids; while other species less different in appearance were quite sterile with the same C.capense.
All the species of the genus Hippeastrum produce hybrid offspring which are invariably fertile.

Lobelia syphylitica and L.fulgens, two very distinct species, have produced a hybrid which has been named Lobelia speciosa, and which reproduces itself abundantly.