Horizontal classification separates ancestral from descendent groups and unites contemporaneous groups, or those in a similar stage of evolution, if they are derived from a common ancestry. Vertical classification unites ancestral and descendent groups and separates contemporaneous groups that are diverging from a common ancestry. It is sometimes stated that these are mutually exclusive principles and that classification must be based on one or the other, but in fact neither can be followed consistently and any classification necessarily combines both methods. The most ardent exponent of vertical classification finds it necessary to separate ancestral and descendent genera, for instance. Each point of view has its advantages and its dangers.
Horizontal classification is usually easier and more objective.
The earlier horses and tapirs, for instance, have more in common than have the earliest and latest horses, and it would be much easier to define and to recognize a horizontal group containing both primitive horses and tapirs than a vertical group containing all the horses and excluding all the tapirs. The horizontal arrangement may also be more stable, because the relative ease of recognition makes it less likely to be disturbed by later discovery, whereas most evolutionary phyla are difficult to distinguish and require considerable material for their proper recognition. Vertical groupings often require extensive rearrangement following new discoveries or new studies that modify ideas of the details of phyletic descent. The principal advantage of vertical, and disadvantage of horizontal, classification is that the former is more in accord with the whole conception of evolutionary descent. Its principal proponents, such as the late H. F. Osborn, therefore, sometimes speak of it as evolutionary classification, as opposed to the more static and historically pre-evolutionary, more strictly Linnaean, horizontal system.