most instructive to
compare--but I have not space here to enter on details--the evidence
advanced by our best botanists on the question whether certain doubtful
forms should be ranked as species or varieties, with the evidence from
fertility adduced by different hybridisers, or by the same author,
from experiments made during different years. It can thus be shown that
neither sterility nor fertility affords any clear distinction between
species and varieties; but that the evidence from this source graduates
away, and is doubtful in the same degree as is the evidence derived from
other constitutional and structural differences.

In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in
the first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing that close interbreeding lessens fertility, and, on
the other hand, that an occasional cross with a distinct individual or
variety increases fertility, that I cannot doubt the correctness of this
almost universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids
will generally be fertilised during each generation by their own
individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gartner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flowe