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 Leguminos , 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
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. It can thus be shown that neither sterility nor
fertility affords any clear distinction between species and varieties;
but that the evidence from this source graduates away, and is doubtful
in the same degree as is the evidence derived from other constitutional
and structural differences.
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