be crossed with facility, but produce utterly sterile hybrids; others can be
crossed with extreme difficulty, but the hybrids when produced are moderately
fertile. I am not aware, however, of any instance quite like this of the
maize, namely, of a first cross made with difficulty, but yielding perfectly
fertile hybrids. (16/14. Mr. Shirreff formerly thought ('Gardener's Chronicle'
1858 page 771) that the offspring from a cross between certain varieties of
wheat became sterile in the fourth generation; but he now admits ('Improvement
of the Cereals' 1873) that this was an error.)

The following case is much more remarkable, and evidently perplexed Gartner,
whose strong wish it was to draw a broad line of distinction between species
and varieties. In the genus Verbascum, he made, during eighteen years, a vast
number of experiments, and crossed no less than 1085 flowers and counted their
seeds. Many of these experiments consisted in crossing white and yellow
varieties of both V. lychnitis and V. blattaria with nine other species and
their hybrids. That the white and yellow flowered plants of these two species
are really varieties, no one has doubted; and Gartner actually raised in the
case of both species one variety from the seed of the other. Now in two of his
works (16/15. 'Kenntniss der Befruchtung' s. 137; 'Bastarderzeugung' s. 92,
181. On raising the two varieties from seed see s. 307.) he distinctly asserts
that crosses between similarly-coloured flowers yield more seed than between
dissimilarly-coloured; so that the yellow-flowered variety of either species
(and conversely with the white-flowered variety), when crossed with pollen of
its own kind, yields more seed than when crossed with that of the white
variety; and so it is when differently coloured species are crossed. The
general results may be seen in the Table at the end of his volume. In one
instance he gives (16/16. 'Bastarderzeugung' s. 216.) the following details;
but I must premise that Gartner, to avoid exaggerating the degree of sterility
in his crosses, always compares the MAXIMUM number obtained from a cross with
the AVERAGE number naturally given by the pure mother-plant. The white variety