of their mongrel offspring, is, on my theory, of equal importance
with the sterility of species; for it seems to make a broad and clear
distinction between varieties and species.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of those
two conscientious and admirable observers, Koelreuter and Gaertner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Koelreuter 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 {247} them as varieties. Gaertner,
also, makes the rule equally universal; and he disputes the entire
fertility of Koelreuter's ten cases. But in these and in many other cases,
Gaertner 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
Gaertner 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 Gaertner 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 Gaertner 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 doub