Something analogous occurs in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on a fourth species, when thus grafted were rendered barren.

On the other hand, certain species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots.

We are reminded by this latter fact of the extraordinary cases of Hippeastrum, Passiflora, etc.

, which seed much more freely when fertilised with the pollen of a distinct species than when fertilised with pollen from the same plant.

We thus see that, although there is a clear and great difference between the mere adhesion of grafted stocks and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species.