ABSTRACT
Many of the practical difficulties of strict phylogenetic classification result from the fact that phenotypic evolution, which is usually a slow, progressive process, is greatly accelerated in certain groups at particular times, most notably, of course, in the well-known 'adaptive radiations'. A major change in the mode of life will expose organisms to strong selection operating in quite new directions, in contrast to those phases during which selection operates mainly as a 'stabilizing' force. The general principle of phylogenetic classification is to make all groups monophyletic and to establish the category of a group by the age of its common ancestral species—measured in years or in generations. The major difficulties in making a phylogenetic system for fossils would not however be eliminated altogether by such a system; they would recur within the system for each geological period, on a smaller scale, and mainly affecting the categories at about genus level.
