Gartner, also, makes the rule equally universal; and he disputes the entire fertility of Kolreuter's ten cases.

But in these and in many other cases, Gartner is obliged carefully to count the seeds, in order to show that there is any degree of sterility.

He always compares the maximum number of seeds produced by two species when first crossed, and the maximum produced by their hybrid offspring, with the average number produced by both pure parent-species in a state of nature.

But causes of serious error here intervene: a plant, to be hybridised, must be castrated, and, what is often more important, must be secluded in order to prevent pollen being brought to it by insects from other plants.

Nearly all the plants experimented on by Gartner were potted, and were kept in a chamber in his house.