ientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
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 first crossed,
and the maximum produced by their hybrid offspring, with the average
number produced by both pure parent-species in a state of nature. But
causes of serious error here intervene: 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 experimented on by Gartner were potted,
and 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 repeatedly
crossed some forms, such as the common red and blue pimpernels
(Anagallis arvensis and coerulea), which the best botanists rank as
varieties, and found them absolutely sterile, we may doubt whether many
species are really so sterile, when intercrossed, as he believed.

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, arrived at diametrically