 As a solution to the variation question, Darwin developed his “provisional hypothesis” of pangenesis, which he presented the year after the appearance of the Fleeming Jenkin review in his two-volume Variation of Plants and Animals Under Domestication (1868, second edition 1875; Olby 2013). Although this theory had been formulated independently of the Jenkin review (Olby 1963), in effect it functioned as his reply to it. This offered a causal theory of variation and inheritance through a return to a theory resembling Buffon’s theory of the organic molecules of the previous century (see entry on  evolutionary thought before Darwin  section 3.2). Invisible material “gemmules” were presumed to exist within the cells, and according to theory, these were subject to possible external alteration by environment and circumstance. The gemmules were then shed continually into the blood stream (the “transport” hypothesis) and assembled by “mutual affinity for each other, leading to their aggregation into buds or into the sexual elements” (Variation 1875: vol. 2: 370). In this form they were then transmitted—the details were not explained—by sexual generation to the next generation to form the new organism out of “units of which each individual is composed” (ibid.). In Darwin’s view, this hypothesis united together numerous issues into a coherent and causal theory of inheritance and explained the basis of variation. It also explained how use-disuse inheritance, a theory which Darwin never abandoned, could work.