It is known that with our domestic animals certain
individuals are sexually incompatible, and will not produce offspring,
although fertile with other individuals. (6/3. I have given evidence on
this head in my 'Variation of Animals and Plants under Domestication'
chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a
case which bears more closely on our present one, as it shows that in
the genus Nicotiana the varieties differ in their sexual affinities.
(6/4. 'Das Geschlecht der Pflanzen, Zweite Fortsetzung' 1764 pages
55-60.) He experimented on five varieties of the common tobacco, and
proved that they were varieties by showing that they were perfectly
fertile when reciprocally crossed; but one of these varieties, if used
either as the father or the mother, was more fertile than any of the
others when crossed with a widely distinct species, N. glutinosa. As the
different varieties thus differ in their sexual affinities, there is
nothing surprising in the individuals of the same variety differing in a
like manner to a slight degree.
Taking the plants of the three generations altogether, the crossed show
no superiority over the self-fertilised, and I can account for this fact
only by supposing that with this species, which is perfectly
self-fertile without insect aid, most of the individuals are in the same
condition, as those of the same variety of the common pea and of a few
other exotic plants, which have been self-fertilised for many
generations.
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