It is the duty of the exploring botanist to expound, not to confound, nature: to bring to light what he meets with, not to impose his own ideas about what ought to be there. Consequently he should not shrink from accepting the fact that, despite heredity, variations occur and lead to the production of new types.

The criteria of a species in Rubus are that it possesses constant and inheritable characters running all through the plant and distinguishing it from all other species. Alongside this assemblage of characters, species are liable to vary from one generation to another in certain characters of a minor kind that are often found also in related species. This faculty of variation does not invalidate the conception of a true breeding species. On the contrary, it is provided for and ensured in the mechanism of reproduction; it may be regarded as an asset, not a disability to the species. As in other groups the species of Rubus differ in size, that is in degree of distinctness; they may be different in mode of origin and in mode of reproduction. They may differ again in their behaviour; they may be able to cross with certain species but not with others; the cross may be sterile, or it may be fertile from the first, or, sterile at first, it may in time become fertile, or even very fertile.

Because of the fluctuating variations referred to, which are controlled to some extent by genes and are liable to be altered in any generation, the characters of the species may not all be present together in every individual; a few will be missing here or there, perhaps occasionally in forms occupying geographically separated areas. If, then, an ice-sheet or an invading sea blots out a part of the population of that species, the parts that remain may not be able to breed together as one unit and may develop along rather different lines. It is possible that a few geographical races may exist of some Rubus species in Wales and south Devon as compared with Sussex and Kent or the west of France.