Chap, VIII. 
STEKILITY. 
247 
them as varieties. Gartner, also, makes the rule 
equally universal; and he disputes the entire fertility 
of Kdlreuter’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 pro¬ 
duced 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 in¬ 
sects from other plants. Nearly all the plants experi- 
mentised on by Gartner were potted, and apparently 
were kept in a chamber in his house. That these pro¬ 
cesses are often injurious to the fertility of a plant can¬ 
not be doubted; for Gartner gives in his table about a 
score of cases of plants which he castrated, and artifi¬ 
cially fertilised with their own pollen, and (excluding 
all cases such as the Leguminosse, 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 pim¬ 
pernels (Anagallis arvensis and coerulea), which the 
best botanists rank as varieties, absolutely sterile to¬ 
gether; 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. 
