Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he unhesitatingly ranks them
as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these
and in many other cases, Gartner is obliged carefully to count the
seeds, in order to show that there is any degree of sterility. He
always compares the maximum number of seeds produced by two species
when crossed and by their hybrid offspring, with the average number
produced by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced: a plant to
be hybridised must be castrated, and, what is often more important,
must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimentised on by
Gartner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gartner gives in his table about a score
of cases of plants which he castrated, and artificially fertilised
with their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some
degree impaired.
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