t whether many other species are really so
sterile, when intercrossed, as Gaertner believes. {248}

It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no better
evidence of this can be required than that the two most experienced
observers who have ever lived, namely, Koelreuter and Gaertner, should have
arrived at diametrically opposite conclusions in regard to the very same
species. It is also 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
Gaertner 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
{249} 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 flower