 Of central importance in analyzing this complex professional reception was the role assigned to normal individual variation and its causes. In the initial public presentation of his theory, Darwin had relied on the novel claim that small individual variations—the kind of differences considered by an earlier tradition as merely “accidental”—formed the raw material upon which, by unlimited addition through the action of natural selection, major changes could be produced sufficient to explain the origin and subsequent differences in all the various forms of life over time. Darwin, however, left the specific causes of this variation unspecified beyond some effect of the environment on the sexual organs. Variation was presented in the Origin with the statement that “the laws governing inheritance are quite unknown” (Darwin 1859: 13). In keeping with his commitment to the gradualism of Lyellian geology, Darwin also rejected the role of major “sports” or other sources of discontinuous change in this process.