s," _Arch. of Philos.,
Psych. and Sci. Methods_, No. 1, New York, 1905; summarized in his
_Educational Psychology_, Vol. III, pp. 247-251, New York, 1914.
Measured on a scale where 1 = identity, he found that twins showed a
resemblance to each other of about .75, while ordinary brothers of about
the same age resembled each other to the extent of about .50 only. The
resemblance was approximately the same in both physical and mental
traits.

[37] The quotations in this and the following paragraph are from
_Thorndike's Educational Psychology_, pp. 304-305, Vol. III.

[38] _Biometrika_, Vol. III, p. 156.

[39] "William of Occam's Razor" is the canon of logic which declares
that it is unwise to seek for several causes of an effect, if a single
cause is adequate to account for it.

[40] Schuster, Edgar, _Eugenics_, pp. 150-163, London, 1913.

[41] _Educational Psychology_ (1914), Vol. III, p. 235.

[42] Cobb, Margaret V., _Journal of Educational Psychology_, viii, pp.
1-20, Jan., 1917.

[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 mor