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xix Besides these differences occasioned by accidental circumstances of growth, all plants are apt to exhibit permanent variations from the general form, due to the operation of more obscure causes, giving rise to what are termed varieties. These varieties are, in perennial plants, capable of propagation by subdivision of the root, or from slips or cuttings; but their seed usually reproduces the normal form, though it is yet a doubtful and disputed point whether they may not in some instances be permanently propagated by seed: they often diverge so greatly from the typical plant, that they are apt to be regarded by the ordinary observer as distinct, and even our most experienced botanists are sometimes unable to decide upon their real character.

Among our native plants, the groups of Roses, Brambles, and Willows are remarkable instances of this obscurity regarding specific distinctions. Of upwards of seventy Willows figured and described in the present work, certainly not more than fourteen or fifteen can be satisfactorily distinguished as species. The twenty-four Roses are probably varieties of not more than five distinct plants; and the species of Rubus, which have been multiplied by some writers to a far greater extent than in this book, are perhaps referable to four specific types. The confusion naturally existing between species and varieties has been unfortunately greatly increased, by the anxiety of the students of local floras to extend the apparent field of their labours by the addition of new species,—an object more easily attained in a well-explored country by the subdivision of well-known groups, than by the discovery of really new plants, and a habit the more to be deplored, as it tends to destroy that simplicity of arrangement which is the only object of classification.

Whether species have any definite limits in Nature, is a question foreign to the scope of the present work. As the term is applied in Botany, it may be generally defined as an assemblage of plants possessing a certain similarity in all essential points of structure, capable of being permanently propagated by seed, and