The Law of Adaptive Radiation

ONE of the essential features of divergent evolution as conceived in the branching system successively developed by Lamarck, Darwin, Huxley, and Cope has been termed by the writer " adaptive radiation." This term seems to express most clearly the idea of differentiation of habit in several directions from a primitive type, as shown in the accompanying diagrams. The law is a familiar one ; it results in the formation of analogous radii in different groups of animals. The first comprehensive illustration of the law known to the writer is that under the headings " Homologous Groups " and " Heterology," in Cope's paper of i868 on the "Origin of Genera," reprinted in the Orzrin of the Fittest (pp. 95-io6). This brilliant essay is marred only by great confusion in the use of terms; but the parallelisms in unrelated groups of amphibians and of mammals such as marsupials and placentals, as first observed by Owen, are clearly brought out. In the present paper citations from earlier essays of my own may be given bearing upon gnci-erl adaplive radiation and the independent production of analogous radii under the convergent