 There were also alternatives to the Mendelian view of heredity. The biometric approach, going back to the work of Francis Galton (1822–1911) and further developed by W.F.R. Weldon (1860–1906) and Karl Pearson (1857–1936), used advanced statistical tools to investigate heredity in terms of correlations between characters in successive generations in limited populations (Porter 1986: Ch. 9). Weldon and Pearson adhered to an orthodox interpretation of Charles Darwin’s (1809–1882) view that evolution proceeded by natural selection acting on gradual variation. Mendelians, instead, focused on discontinuous variation (Bowler 1992; Gayon 1998). Wilhelm Johannsen (1857–1927) had conducted experiments with “pure lines”, in which he selected the plus or minus variants of quantitative characters such as seed size in the offspring of self-fertilized beans (Johannsen 1903). Based on his results, Johannsen distinguished between fluctuating variation, which was due to contingent influences of the environment, and variation between types, which remained stable unless changed in a discontinuous manner through mutation (Roll-Hansen 2009). Around that time, Hugo de Vries (1848–1935; 1901–1903) suggested the concept of mutation to account for the spontaneous emergence of new stable types. Even though many did not follow de Vries’ idea of large-scale changes affecting several characters at once, giving rise to a new species instantaneously, most Mendelians came to think that the character variation they observed occurred spontaneously through changes in factors as units of the hereditary material (Bowler 1992: Ch. 8).