 Initially, some Mendelians doubted the purity of gametes and concluded that the maternal and paternal gametes “contaminated” each other regarding the factors by which they differed, thus allowing for continuous variation. However, various researchers were able to resolve the question by showing how several factors affected the same quantitatively characterized character in cumulative manner (Carlson 1966: Chs. 4–5; Darden 1991: Chs. 6–7). Around 1910, models of “multi-factorial inheritance” explained modes of inheritance of continuously varying characters based on discontinuous genetic variation. These and other developments led to a convergence of biometric approaches and Mendelism and the emergence of mathematical population genetics in the 1920s and 30s, a field that laid the foundations for genetic models of the effects of selection and other processes of evolution (Allen 1978: Ch. V, Part II; Bowler 1989b: Ch. 11; Provine 1971; entries on  population genetics  and  ecological genetics).