ly nine others) on a dwarf
maize bearing yellow seed[225] with pollen of a tall maize having red
seed; and one head alone produced good seed, only five in number.
Though these plants are monoecious, and therefore do not require
castration, yet I should have suspected some accident in the
manipulation had not Gaertner expressly stated that he had during many
years grown these two varieties together, and they did not
spontaneously cross; and this, considering that the plants are
monoecious and abound with pollen, and are well known generally to
cross freely, seems explicable only on the belief that these two
varieties are in some degree mutually infertile. The hybrid plants
raised from the above five seed were intermediate in structure,
extremely variable, and perfectly fertile.[226] No one, I believe, has
hitherto suspected that these varieties of maize are distinct species;
but had the hybrids been in the least sterile, no doubt Gaertner would
at once have so classed them. I may here remark, that with undoubted
species there is not necessarily any close relation between the
sterility of a first cross and that of the hybrid offspring. Some
species can 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 with natural species, namely, of
a first cross made with difficulty, but yielding perfectly fertile
hybrids.

The following case is much more remarkable, and evidently perplexed
Gaertner, 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 Gaertner actually raised in the case
of both species one variety from the seed of the other. Now in two of
his works[227] he distinctly asserts that crosses between
similarly-coloured flowers yield more seed than between