lineage during a given time period. For example, in studying the evolution of horse (Equus) from Eohippus, he found that eight genera were given rise over the course of approximately 45 million years, which gives a rate of 0.18 genera per million years.

J.B.S. Haldane proposed the first standard unit for morphological evolutionary rate, the ''darwin'' (d), which represents a change in measurable trait by a factor of e (the base of natural logarithms)