tever
amount of infertility may at first exist could be eliminated by careful
selection, if the crossed races were bred in large numbers and over a
considerable area of country. 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. 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