[26] Its eugenics is to be effected through the mental exertion of mothers. And we have lately been in correspondence with a western attorney who is endeavoring to form an association of persons who will agree to be the parents of "willed" children. By this means, he has calculated (and sends a chart to prove it) that it will require only four generations to produce the Superman. [27] _Life and Letters of Charles Darwin_, Vol. I, p. 302, New York, 1897. The letter is dated 1844. [28] Goddard, H. H., _Feeble-mindedness_, p. 359. New York, the Macmillan Company, 1914. [29] For a review of the evidence consult an article on "Telegony" by Dr. Etienne Rabaud in the _Journal of Heredity_, Vol. V, No. 9, pp. 389-400; September, 1914. [30] It will be recalled that the coefficient of correlation measures the resemblance between two variables on a scale between 0 and-1 or +1. If the correlation is zero, there is no constant relation; if it is unity, any change in one must result in a determinate change in the other; if it is 0.5, it means that when one of the variables deviates from the mean of its class by a given amount, the other variable will deviate from the mean of its class by 50% of that amount (each deviation being measured in terms of the variability of its own class, in order that they may be properly comparable.) [31] Sidis, Boris, M.A., Ph.D., M.D., "Neurosis and Eugenics," _Medical Review of_ _Reviews_, Vol. XXI, No. 10, pp. 587-594, New York, October, 1915. A psychologist who writes of "some miraculous germ-plasm (chromatin) with wonderful dominant 'units' (Chromosomes)" is hardly a competent critic of the facts of heredity. [32] In a letter to the _Journal of Heredity_, under date of August 4, 1916. [33] Galton, Francis, _Inquiries into Human Faculty_, p. 167, London, 1907. [34] Woods, Frederick Adams, _Heredity in Royalty_, New York, 1906. [35] _Op. cit._, pp. 170-171. [36] Thorndike, E. L., "Measurements of Twins," _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.