On the Reception of Johannsen's Pure Line Theory: Toward a Sociology of Scientific Validity

There have been ongoing controversies between Barnes and Roll-Hansen as to the rationality of Johannsen's pure line theory around 1910. While Roll-Hansen argues that the victory of Mendelism and pure line theory over biometry is the result obtained from the methodical application of scientific methods by the scientific community, Barnes argues that Roll-Hansen's criticism represents a massive leap beyond the findings he makes available. According to Barnes, Roll-Hansen's study of an individual scientist (Johannsen) does not enable him to extend his rationalistic argument to the biological community in general. This paper contributes to this ongoing debate by adding a detailed description of the social process of mutual persuasion among scientists, and thereby attempts to show why and how pure line theory comes to be accepted by the biological community. By presenting four cases of biometricians who were converted to Johannsen's theory, I specifically argue that the natural world plays a much more important role in the validation of pure line theory than MacKenzie and Barnes suppose in their externalist-sociological studies of the biometry-Mendelism controversy.