Evolution by natural selection includes two main steps: the generation of heritable variations (e.g., mutations) and the differential proliferation of the variants in the environment. When the neo-Darwinists synthesized a modern view of natu- ral selection and genetics in the early 20th century, they spec- ified a simplifying assumption that Darwin (12) had not: that the rates of formation of mutations would be independent of exposure to selective environments (e.g., see reference 47). Thus, evolution, and the mutations driving it, should be con- stant and gradual. That some spontaneous mutations form independently of interaction with the environment is certainly true (42, 46, 52; see also many subsequent papers). These form before an organism encounters a selective environment, with a definable relationship to cell divisions ("growth-dependent mutations"), probably because many result from DNA repli- cation errors. However, work with several microbial assay sys- tems indicates the existence of additional mutation pathways that appear to be induced in response to the environment (reviewed in references 16, 58, and 60). These mutation mech- anisms, called stationary-phase or stress-induced mutation, op- erate specifically under growth-limiting stress and may some- times produce mutations that confer a growth advantage in the growth-limiting environment, called adaptive mutations. The problem is, are they really something different from growth- dependent mutations?