a logarithmic unit measuring the rate of evolution in characteristics of organisms. If the measurement of a characteristic (such as length, for example) changes from a to b in y million years, then the rate of evolution is (ln b - ln a)/y darwins. (Technically, since the difference in natural logarithms is dimensionless, the darwin is a specialized name for the inverse megayear Ma-1).
Also denotes a logarithmic unit of evolution: any quantifiable attribute of an organism, esp. size of physical structures, changing by a factor of e (≈ 2.7182818…) per million years, is evolving at the rate of 1 darwin.