The evidence is at least as good as that from which we believe in the sterility of a multitude of species.

The evidence is also derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction.

Gartner kept, during several years, a dwarf kind of maize with yellow seeds, and a tall variety with red seeds growing near each other in his garden; and although these plants have separated sexes, they never naturally crossed.

He then fertilised thirteen flowers of the one kind with pollen of the other; but only a single head produced any seed, and this one head produced only five grains.