Intellectual Merit. While Darwins theory of evolution emphasized chance in the sense that variation should not be purposeful or lead directly to adaptation, it did not incorporate probability in the mathematical form of frequency distributions. More crucially, Darwin rejected any idea that chance in his theory involved absence of causes. Through the rest of the nineteenth century, the claim that a biological theory depended on chance in this sense was usually a basis for criticizing it. But the more-or-less simultaneous appearance in the early 1890s of a biometric science of heredity and of the systematic study of discontinuous variation (including, by 1900, mutation theory and Mendelism) made variation an explicit object of biological attention and theorizing. Such approaches involved chance in a much more central role. This proposal is part of a larger project on chance in the twentieth-century sciences. It addresses two important topics associated with chance and variation in early twentieth century biology. One is the relation between biometry and continuous or blending inheritance. It appears that blending as a theory of heredity was endorsed by almost nobody in this period. The concept of blending inheritance was introduced in 1930 by R.A. Fisher as background for his own analysis of natural selection, and also to distance himself from biometric predecessors such as Pearson. The biometricians, in fact, seem never to have argued on principle against inheritance of discrete hereditary factors. There is instead a continuous history, beginning with Galtons version of Pangenesis, of deriving the statistics from a particulate theory, or at least of using statistics to define the terms of a particulate theory. This point leads to the second main argument. Just as biometry was seen as compatible with particulate inheritance, Mendelism was in important ways statistical. The distinction between these rival programs was more subtle than historians have generally appreciated. Mendelian genetics as a kind of random process, with its segregation of genes and its mutations, was in effect a statistics of variation, and, conversely, the statistics of the biometricians involved the assumption of random processes, even in the hereditary factors. The controversy of biometry and Mendelism was real, of course, but it may be appropriate in some respects to see them as rival versions of a common program rather than fundamentally incompatible ones. One aspect that united them was their growing reliance on chance. Broader Impacts. The work is intended to lead in the longer term to a short book for a relatively wide audience on the immense role of chance in science in the twentieth century. Such work will contribute to popular understanding regarding a scientific issue that has remained a subject of lively discussion and debate, especially with respect to biological evolution.
