opposite conclusions in regard to some
of the very same forms. 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 observer from experiments made
during different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and varieties.
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
increases, but generally decreases greatly and suddenly. With respect
to this decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is often
transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I
believe that their fertility has been diminished in nearly all these
cases by an independent cause, namely, by too close interbreeding. I
have made so many experiments and collected so many facts, showing on
the one hand that an occasional cross with a distinct individual or
variety increases the vigour and fertility of the offspring, and on
the other hand that very close interbreeding lessens their vigour and
fertility, that I cannot doubt the correctness of this conclusion.
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, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower; and
this would probably 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,