On The Origin of Species by Charles Darwin Chapter 9 Page 79

know not; but the forms experimented on are ranked by Sagaret, who mainly founds his classification by the test of infertility, as varieties, and Naudin has come to the same conclusion.

The following case is far more remarkable, and seems at first incredible; but it is the result of an astonishing number of experiments made during many years on nine species of Verbascum, by so good an observer and so hostile a witness as Gartner: namely, that the yellow and white varieties when crossed produce less seed than the similarly coloured varieties of the same species.

Moreover, he asserts that, when yellow and white varieties of one species are crossed with yellow and white varieties of a DISTINCT species, more seed is produced by the crosses between the similarly coloured flowers, than between