The identification of the selective forces acting on natural variation in a population of organisms may be a formidable task. Only a few such identifications have been made, as in the studies of Cain and Sheppard (1954 et seq., summarized by Ford, 1964) and others on polymorphism in Cepaea snails, or the studies of Kettlewell (summary: Kettlewell, 1961) on industrial melanism in moths. The wing pigmentation of pierid butterflies presents many opportunities for this sort of analysis. Much natural variation in this system has been noted in the genera Pieris and Colias (Bowden, 1961, 1963; Gerould, 1923; Remington, 1954). These insects deposite two kinds of pigment in their wing scales: melanin, which is black, and pteridines, which may be white, yellow, orange, or red. The wings of different Colias species may possess any of the latter shades as