Of course these factors are not of equal importance; some of them produce large changes and some, as far as can be told, are of minor significance. The factors, moreover, undergo large changes from time to time, thus producing mutations; and it is probable small changes as well, the evidence for which requires greater refinements of method than is usual among those using the pedigree method.

A Critique of the Theory of Evolution, by Thomas Hunt Morgan, professor of experimental zoölogy in Columbia University. Princeton University Press, 1916. This book gives the best popular account of the studies of heredity in Drosophila. The advanced student will find The Mechanism of Mendelian Heredity (New York, 1915), by Morgan, Sturtevant, Müller, and Bridges, indispensable, but it is beyond the comprehension of most beginners.