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.
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