The Scala naturae revisited: evolutionary scales and anagenesis in comparative psychology.

Recent suggestions that evolutionary scales have a place in theorization about the evolution of behavior have been based on the concept of anagenesis, formerly associated with notions of biological progress. An associated concept is that of grades, often used as units of anagenetic advance. Advocates of anagenetic analysis in comparative psychology cite the writings of biologists Bernard Rensch, Julian Huxley, George Gaylord Simpson, and Stephen Jay Gould to support the usefulness of anagenesis but treat the positions of each of these theorists as if they were the same. In fact, they differ considerably in their definition of anagenesis and in its application to specific issues in evolution. The anagenetic approach is criticized as axiological and frequently anthropocentric. Although the formation of grades can be useful, a sequence of grades must not be assumed to represent historical stages in the evolution of specific structures or behaviors.