 The pangenesis theory, although not specifically referred to, seems to be behind an important distinction he inserted into the fifth edition of the Origin of 1869 where he made a direct reply to the criticisms of Jenkin. In this textual revision, Darwin distinguished “certain variations, which no one would rank as mere individual differences”, from ordinary variations (Origin 1869: 105) [Peckham 1959 [2006: 178–179]]). This revision shifted Darwin’s emphasis away from his early reliance on normal slight individual variation, and gave new status to what he termed by the sixth edition of 1872 “strongly marked” variations. The latter were now the form of variation to be given primary evolutionary significance, and presumably this was more likely to be transmitted to the offspring, although details are left unclear. In this form it presumably could be maintained in a population against the tendency to swamping by intercrossing. Darwin’s struggles over this issue defined a set of problems that British life scientists in particular were to deal with into the 1930s. The debates over variation placed Darwinism in a defensive posture that forced its supporters into major revisions in the Darwinian research program (Gayon 1998; Vorzimmer 1970). The long period between 1870 and 1930, sometimes characterized as the “Eclipse” of Darwinism, or at least of his natural selection theory, was the outcome of these debates (Bowler 1983, 2013a).