AN EXPLANATION FOR COPE'S RULE

Cope's Rule is the name customarily applied to the widespread tendency of animal groups to evolve toward larger physical size. According to Kurten (1953), this paleontologic rule of evolution "is second in repute only to 'Dollo's Law' of . . .irreversibility." Although the rule was never concisely formulated by Cope, it is generally implicit in his writings (Cope, 1887, 1896). Like many other evolutionary generalizations extracted from the fossil record, Cope's Rule was derived primarily from study of mammalian phylogeny. Writers like Deperet (1909), Newell (1949), and Rensch (1959) have added diverse examples of evolutionary size increase to the mammalian trends discussed by Cope. Because numerous exceptions are known, recognition of the concept as a law has been rejected by most workers. Still, it has been widely upheld as a valid empirical generalization, and of the definitions for "rule" listed by Webster, "a generally prevailing condition," describes it accurately. Most modern interpreters of Cope's Rule have attributed its validity solely to certain fundamental advantages of size increase, at least one of which is alleged to have operated within most evolutionary lineages (Newell, 1949, p. 122-123; Kurten, 1953, p. 105; Simpson, 1953, p. 151; Rensch, 1959, p. 210-211; Gould, 1966b, p. 1138). Most of the proposed advantages of evolutionary size increase have been reviewed by Newell (1949), Kurten (1953), Rensch (1959), and Gould (1966a). Among the more salient are: improved ability to capture prey or ward off predators, greater reproductive success, increased intelligence (with increased brain size), better stamina, expanded size range of acceptable food, decreased annual mortality, extended individual longevity, and increased heat retention per unit volume. Certainly when evolutionary size increase occurs, it is in response to selection pressure resulting from one or more advantages. As Bonner (1968) pointed out, however, a net trend toward increased size within a higher taxon usually produces an increase in mean and maximum animal size, but not necessarily in minimum animal size. Selection pressure favoring size decrease is not rare but only less common than pressure favoring size increase. Reliance solely on inherent advantages of larger size to explain the high incidence of size increase suffers from deficiencies resembling some of those attached to the discarded concept of orthogenesis. The "inherent advantage" idea attempts only to account for directional evolution, without reference to limits. Clearly when size increase occurs in a