ile 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. Moreover, as Gartner during several years
repeatedly crossed the primrose and cowslip, which we have such good
reason to believe to be varieties, and only once or twice succeeded in
getting fertile seed; as he found the common red and blue pimpernels
(Anagallis arvensis and coerulea), which the best botanists rank as
varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we
may well be permitted to doubt whether many other species are really so
sterile, when intercrossed, as Gartner believes.

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, Kolreuter and
Gartner, should have arrived at diametrically opposite conclusions
in regard to the very same species. It is also