This paper traces the background to R. A. Fisher's multi-factorial theory of inheritance. It is argued that the traditional account is incomplete, and that Karl Pearson's well-known pre-Fisherian objections to the theory were in fact overcome by Pearson himself. It is further argued that Pearson's stated reasons for not accepting his own achievement has to be seen as a rationalization, standing in for deeper-seated metaphysical objections to the Mendelian paradigm of a type not readily discussed in a formal scientific paper. (...) The apparent, post-Fisherian, continued acceptance of Pearson's objections is presented as an interesting problem for the historian and sociologist. (shrink)