On this subject Focke writes me as follows:--"In a natural group (family, order, genus) showing considerable variability in the structure of the flower, we may expect to find [or do find] a greater number of mules than in a group whose species are only distinguished by differences in the shape of the leaves, or in growth, &c. I do not know, however, which in this connexion of things is the cause and which the effect. A useful ancestral structure of the flower may be conserved by an otherwise varying progeny, on condition that the progress of diversity be not disturbed by frequent intercrossings. [Therefore, if this condition be satisfied, the structure of the flower in different members of the group will continue constant: here the cause of _constancy_ in the flower (however much variability there may be in the leaves, &c.) is its original _inability_ to hybridize.] On the other hand, in species or groups ready to hybridize [or capable of hybridizing], the fixation of a new specific type will require some change in the structure of the flower, and a change considerable enough to alter the conditions of fertilization. [Here the reason of the _in_constancy of the flower in different members of the group is the original _aptitude_ of their ancestral forms to hybridize.] Perhaps there is something in this suggestion, but certainly there are other efficient physiological relations, which are at present unknown. Your theory of physiological selection may serve to explain many difficult facts." _The Importance of Prepotency._--A. Kerner shows by means of his own observations on sundry species of plants which hybridize in the wild state, that they do so very much more frequently if both, or even if only one of the parent forms be rare in the neighbourhood. This fact can only be explained by supposing that, even in species most prone to hybridizing under Nature, there is some degree of prepotency of pollen of the same species over that of the other species; so that where both species are common, it is correspondingly rare that the foreign pollen gets a chance. But if there were no prepotency, the two species would blend; and this Kerner supposes must actually take place wherever two previously separated species, thus physiologically circumstanced, happen to be brought together. (Kerner's paper is published in _Oester. Bot. Zeitschrift_, XXI, 1871, where he alludes to sundry other papers of his own advocating similar views.) The relation of these observations to Jordan's _espèces affines_ is obvious. We have only to suppose that some such slight and constant difference characterizes the sexual elements of these allied varieties as demonstrably characterizes their morphology, and we can understand how pollen-prepotency would keep the forms distinct--such forms, therefore, being so many records of such prepotency.