ABSTRACT
The Manichean classification of organic resemblances divides similarities of form into treacherous, deceptive analogs and guiding, enlightening homologs. The analog is the paleontologist’s only source of experimental material in the testing of any functional hypothesis. The functional theme has its extreme statement–as uncompromising as pure phyleticism and as far from Aristotle’s aurea mediocritas. An adequate theory of functional morphology must explain the constraints on adaptive design by studying how different organisms react to the same selective regime. In moving beyond tenuous inferences from morphology and sedimentary environment, people must study the living organism “most like” the designated fossil. N. Hotton argues that the lower vertebrate classes arose by rapid invasion of new environments by forms that had expanded to an edge of the adaptive zone of ancestral types. New classes may then arise from single clades in virtual absence of competition with parental forms.
