ABSTRACT
Fishes, analyzed as swimming machines, form different grades than do the same fishes judged as feeders. Many higher taxa are not the monophyletic clades that most theories of classification require, but are grades of improvement attained in the same way by many lineages. Clades are the true monophyletic branches of an evolutionary tree. Grades, stages of progress, or levels of organization vary according to the criteria employed, the organs analyzed, and the limits of current knowledge about optimal design and the nature of materials. The primary value of J. S. Huxley’s alternate picture, for all its restriction and subjectivity, is its frank display of a bias contrary to orthodoxy. Evolution, when operating in broad and relatively stable environments, should display some predictable components. Increasing cladistic diversity in successive grades may be predicted from the general supposition that broad improvements in design increase the range of potential variations.
