e 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.

[48] "On the Inheritance of Some Characters in Wheat," A. and G. Howard,
_Mem. Dep. of Agr. India_, V: 1-46, 1912. This careful and important
work has never received the recognition it deserves, apparently because
few geneticists have seen it. While the multiple factors in wheat seem
to be different, those reported by East and Shull appear to be merely
duplicates.

[49] "The Nature of Mendelian Units." By G. N. Collins, _Journal of
Heredity_, V: 425 ff., Oct., 1914.

[50] Dr. Castle, reviewing Dr. Goddard's work (_Journal of Abnormal
Psychology_, Aug.-Sept., 1915) concludes that feeble-mindedness is to be
explained as a case of multiple allelomorphs. The evidence is inadequate
to prove this, and proof would be, in fact, almost impossible, because
of the difficulty of determining just what the segregation ratios are.

[51] In strict accuracy, the law of ancestral inheritance must be
described as giving means of determining the probable deviation of any
individual from the mean of his own generation, when the deviations of
some or all of his ancestry from the types of their respective
generations are known. It presupposes (