Cope’s Rule

A principle proposed by Edward Drinker Cope in 1871, to the effect that species evolve toward larger body sizes. Biologists have pointed out many lineages in which evolution has seemed to follow this rule, including those of horses and humans.

In a paper published in 1997, David Jablonski reported a test of the rule through an extensive study of the evolution of North American clams and snails between 81 and 65 million years ago. He found the rule did not describe the facts.

David Jablonski.
Body-size evolution in Cretaceous molluscs and the status of Cope's rule.
Nature, vol. 385, pages 250 - 252 (16 Jan 1997), doi: 10.1038/385250a0, Letter.