It is the aim of this paper to explain Karl Pearson's rejection of Mendelian genetics.l As an explanatory problem it has two main sorts of interest. First, it has a certain practical interest. Pearson deprived the new genetics of his great mathematical skills, and, it seems reasonable to suppose, thereby materially retarded the development of English population genetics. Second, it has a certain philosophical interest. Here was a man with an excellent record of achievement in the study of heredity, whom with the wisdom of hindsight, we see to have made an incorrect evaluation of an embryonic research program. As students of the dynamics of scientific change, we may wish to study this episode in the hope of finding out something of significance about the ways in which scientists evaluate new and budding rival research programs. Do they use Popperian criteria? Is their judgment affected by professional jealousies and institutional commitments? Let us see how things stand in this particular case.
The problem, of course, is not a new one. And, insofar as there is a consensus I think it is fair to say that the standard view is that the hereditarian research programs of the biometricians Karl Pearson and W. F. R. Weldon, on the one hand, and of the Mendelians, on the other, were not really rival -the one being at the phenomenological level and the other at the physiological level; so that any conflict between the two groups was, logically speaking, a phony conflict. This being so, 1. Karl Pearson (1857Pearson ( -1936) ) was educated as a mathematician, but taking to philosophy of science and social questions, was able to satisfy both interests by extending the work of Francis Galton, and by collaborating with W. F. R. Weldon (1860Weldon ( -1906) ) to create the discipline of biometry. This represented an attempt to croat a new science along the lines advocated in Pearson's Grammar of Science, and also to lay the foundations for a possible program of eugenics (for which, of course, the study of heredity, which was the central feature of biometry, was very important). Both Weldon and Pearson opposed Mendelism, but their own rival research program was curtailed by Weldon's early death in 1906. After Weldon's death, Pearson continued in his opposition, being, in fact, one of the referees whose comments led to Fisher's famous paper "The Correlation between Relatives on the Supposition of Mendelian Inheritance," Trang R. Soc Edinb., 52 (1918), 399-433, being published in Edinburgh rather than in the journals of the