It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility.

It is surprising in how many curious ways this gradation can be shown; but only the barest outline of the facts can here be given.

When pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust.

From this absolute zero of fertility, the pollen of different species applied to the stigma of some one species of the same genus, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the plant's own pollen produces.