We have spoken thus far of the distribution of individuals in a single kind of performance. What does quantitative psychology teach with respect to the combination of performances in a given personality? Is it true, as folk-wishes would have it, that abilities are distributed among us by a law of compensation? Is the slow man’s slowness offset by accuracy? Does the quick learner lose his learning more readily than the slow learner? Is he who excels in arithmetic likely to be surpassed at spelling? The general consideration of these questions, which form the topics of this volume, will be found in the chapter which follows. It will be seen that there is no law of compensation in human ability, however much we may long to find it there. [Illustration: FIG. 2.—Flight of birds, illustrating distribution in ability to fly. (Schematic.) ] As for the origin of talents and defects, psychology teaches that mental endowment in human beings is conditioned by ancestry, just as other traits of organisms are. Mental capacities are inherited through the germ-plasm. A child is gifted (if he is so) for the same reason that he is an Eskimo (if he is one)—because some or all of his ancestors carried those traits in their germ-plasm, and the combination of them in just that way was possible. REFERENCES MEUMANN, E.—_Vorlesungen zur Einführung in die experimentelle Pädagogik_; Engelmann, Leipzig, 1914. SEASHORE, C.—_Measures of Musical Talent_; Columbia Graphophone Company, New York, 1919. STERN, W.—_Die differentielle Psychologie_; Barth, Leipzig, 1911. THORNDIKE, E. L.—_Educational Psychology_; 3 vols. Teachers College, Columbia University, 1913. CHAPTER II THE RELATIONSHIPS AMONG CAPACITIES I. THE COEFFICIENT OF CORRELATION The question is: How are mental capacities mutually related, with regard to amounts of each found in given individuals? Before verifiable facts can be established in a field of knowledge, it is necessary to introduce therein methods of enumeration and measurement. The question above propounded has waited long for answer, because of the great difficulty of applying mathematics to mental phenomena. The answer required first that single functions be accurately scored, and then that a measurement be obtained of the _relationship_ between and among the single functions. It seems well agreed that the quantitative determination of the relationship between and among mental characteristics began with Galton, about 1885. Various scholars have presented discussions of the subject since then, notably Baerwald in 1896, Spearman in 1904, Stern in 1911, Meumann in 1913, and Thorndike in 1913, each of whom summarized the findings up to the time of writing, with original interpretations.