Clines: An Auxiliary Method in Taxonomy

Abstract

While we may hesitate to add to terminology, there are occasions when new terms are a help instead of a burden. I believe this to be so with regard to taxonomie description. The binomial nomenclature is now giving place, in all well-investigated groups, to a trinomial. The third or subspecific name is usually given to geographical groups (subspecies) whether they be geographically isolated or determined only to a zone of intergradation. To the name, a descriptive diagnosis is of course added. Various difficulties arise. In the first place, where is one to draw the line? Should every geographical group which it is possible to distinguish be raised to subspecific rank? In practice, the arbitrary rule is often adopted (e.g. in ornithology) that this should not be done unless 75 % of the population can be distinguished from the neighbouring subspecies. Secondly, should trinomials be reserved for geographical subspecies? Some authors consider that they should ; but others commonly employ them for physiological races, ecological types, paleontological stages, and even for aberrations or the different forms of polymorphic species. It is clear, as Rensch (1934) points out, that too great an extension of the trinomial system defeats its own ends : but how are we to draw the line ? Thirdly, while it is known that many organisms vary geographically and ecologically in a graded way, no recognized terminology exists for these gradients. This is unfortunate for several reasons: (a) when gradation exists within a group, the mere conferring of a subspecific or a specific name gives a false impression of the geographical homogeneity of the group, (b) when intergroup gradation exists, mere naming and description of the separate groups not only does nothing to bring out the gradation in their characters, .though this relation of connection may be at least as important as their relation of separation with regard to difference of characters, but it may even serve to disguise it.