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
most instructive to compare--but I have not space here to enter on
details--the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during
different years.
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