The origin and nature of organismal variation, in all of its manifestations—morphological, behavioral, and physiological—was a primary concern of Charles Darwin from his earliest days as a lifelong “generation theorist.” Consistent with prevailing beliefs of the time, Darwin subscribed to blending inheritance, a central role for environment in creating variation, and to the assumption that nearly all variation is heritable. No distinction was made between genotype and phenotype at the time, nor was there recognition of plasticity as a phenomenon. In this chapter I trace Darwin’s evolving thinking on the nature and significance of variation, including (1) his early speculations on sexual reproduction in driving variation and transmutational change, (2) the role of environmentally induced variation as raw material for natural selection, and (3) his “Provisional hypothesis of Pangenesis” and its reception. I conclude with a discussion of late 19th to early 20th century empirical and theoretical developments in variation and heredity as efforts to improve upon, or abandon, Darwin’s model, and how these played a role in the discovery of the genotype/phenotype distinction and adaptive phenotypic plasticity—bringing the environmental basis of trait variation to the fore in a way that Darwin would have understood and appreciated.