[43] This is not true of the small English school of biometrists, founded by Sir Francis Galton, W. F. R. Weldon and Karl Pearson, and now led by the latter. It has throughout denied or minified Mendelian results, and depended on the treatment of inheritance by a study of correlations. With the progress of Mendelian research, biometric methods must be supplemented with pedigree studies. In human heredity, on the other hand, because of the great difficulties attendant upon an application of Mendelian methods, the biometric mode of attack is still the most useful, and has been largely used in the present book. It has been often supposed that the methods of the two schools (biometry and Mendelism) are antagonistic. They are rather supplementary, each being valuable in cases where the other is less applicable. See Pearl, Raymond, _Modes of Research in Genetics_, p. 182, New York, 1915 [44] Few people realize what large numbers of plants and animals have been bred for experimental purposes during the last decade; W. E. Castle of Bussey Institution, Forest Hills, Mass., has bred not less than 45,000 rats. In the study of a single character, the endosperm of maize, nearly 100,000 pedigreed seeds have been examined by different students. Workers at the University of California have tabulated more than 10,000 measurements on flower size alone, in tobacco hybrids. T. H. Morgan and his associates at Columbia University have bred and studied more than half a million fruit flies, and J. Arthur Harris has handled more than 600,000 bean-plants at the Carnegie Institution's Station for Experimental Evolution, Cold Spring Harbor, L. I. While facts of human heredity, and of inheritance in large mammals generally, are often grounded on scanty evidence, it must not be thought that the fundamental generalizations of heredity are based on insufficient data. [45] For a brief account of Mendelism, see Appendix D. [46] 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. [47] _A Critique of the Theory of Evolution_, by Thomas Hunt Morgan, professor of experimental zooelogy 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, Mueller, and Bridges, indispensable, but it is beyond the comprehension of most beginners.