462 THE SALIX LISTS IN THE 'LONDON CATALOGUE.' than either Andersson or Sir J. D. Hooker. This latter step has produced a confusing result, since under S. phylicifolia Linn, stand lettered as three varieties — a. phylicifolia Linn. auct. b. phylicifoha — nigricans Wimm. c. nigricans Sm. The second of these names is used, not in Wimmer's sense, but " as a convenient designation for the numerous forms which cannot be referred to either of the extremes " ; while under each of the vars. a. and c. are ranged a number of hybrids, an innovation which has a clumsy appearance and is out of Jieeping not only with the rest of the list, but with the whole Catalogue. It may readily be admitted that the numerous forms of S. nigricans and S. phylicifolia run into one another, and on this ground, in any other genus almost, this would be a strong argument for combining the two species in one. But in a genus in which hybridization not un- frequently occurs, and definite lines of demarcation are obliterated by recrossings of the species with one of its hybrid ofi'spring, the many intermediates between type S. nigricans and type 8. i^hyli- cifolia are easily accounted for. There is a similar case in the regular series of gradations that occur between S. alba L. and S.fragilis L. As Dr. White has observed, in describing S. viridis Fr., «' the hybrid, as met with, more frequently shows a departure " from typical viridis ** towards eithev fragilis or alba, till finally it is almost impossible to separate it from one or other of these species " [Journ. Linn. Soc. xxvii. pp. 372, 373). There are two alternative views to choose between, either of which would account for the present state of things : one is to regard S. alba and S. fragilis as old-established species which by hybridization and innumerable crossings and recrossings between the types and the hybrids have given us in the present day every variation ; or to regard these two as one original species, which has gradually diverged in two direc- tions, and presents to us now two extreme forms, each of them very common, well-marked, and usually constant in character, accompanied by some variations mostly lying between these two forms, but as a rule much scarcer than either of them. Even if this latter theory should be the more acceptable alternative, the time would surely have come when the two extreme forms, in this case S. alba and S. fragilis, would deserve recognition as distinct types and the specific rank which has always been accorded them. In the same way, on either theory, it is more convenient, if not more true, to regard S. nigncans and S. phylicifolia as specific types. In Dr. White's list, that is, in Andersson's specific order, S. viminalis L. has place between S. Arbuscula L. and S. lanata L., and the S. purpurea group brings up the rear. Searching for a reason why the lowland S. viminalis should be inserted between these two alpine species, I perceived that, though it has little in common with S. Lanata beyond the long style, and with S. Arbuscula besides the short pedicel and long narrow nectary, 8. viminalis is so closely allied to 8. Lapjjonum L. in every detail of the catkin, and also in the foliage, that the latter may be regarded as the alpine correlative
