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.

For a brief account of Mendelism, see Appendix D.