Robur, but certain varietal forms of it have been distinguished, of which the commonest in this country are Q. pedunculata, a variety with the female flowers on long peduncles, and Q. sessiliflora, with the female flowers on short peduncles; but although numerous attempts have been made to define these forms, and while small differences in the petioles, lobing of the leaves, and the wood, etc., have been insisted upon at various times by observers, it appears that the two varieties graduate into one another by intermediate forms. In England, the vaxiety pedunculata is the commonest over the country generally, but in the hilly districts of North Wales and the north of England the variety sessiliflora is said to prevail. Similarly, on the Continent the latter variety is found at higher elevations than the former, though its area of occurrence is more restricted. This pronounced variability of the oak was commented upon by the late Charles Darwin, who points out, in the Origin of Species, that more than a dozen species have been made by a certain author out of what other botanists regard as mere varieties of the common oak.
De Candolle, who made a special study of this group, found the variations so enormous that, although he made something like 300 species, he decided that the majority of these were merely provisional; and he concluded, as others have done, that we have, in the numerous varieties of the species of this old genus Quercus, series of incipient species. If the connecting forms were to die out, leaving certain varieties more isolated than they