Let us be more concrete in this matter. If I catch the drift of biological discussion (a hazardous assumption, it may be, for the layman in biology to make), no current and generally accepted doctrine of heredity is able to trace in an unbroken series of structures or events the details of the parental organism through the stages of reproduction to the corresponding details of the offspring. The nuclear and nonnuclear substances that are supposed to represent the "vehicle" of heredity do not, I think (except, perhaps, in a few cases) show variations that represent and correspond to the likeness or difference in given characters as these appear in parent and offspring (e. g., differences in height, in shape of leaf, or in color of hair). If, at some future time, these variations are discovered, then they will, I suppose, represent or correspond to mental as well as physical likeness and difference. At present, however, degree of likeness in blood-relations must be derived from description or measurement of corresponding characters or qualities to be observed in succeeding generations. And the point at which we are here aiming is this: the establishment of inheritance of these qualities, whether physical or mental, must, in principle, rest upon one and the same basis. The inheritance of eye-color and the inheritance of memory-type, the inheritance of an "athletic build" and the inheritance of a bad temper are facts of the same order, and similar methods may be laid under prescription for their establishment.
The great difficulty lies here: how are the characters, mental and physical, to be conceived? and how are they to be described and measured?
We have just seen that upon this question of analysis and measurement, quite apart from the problem of mechanism, the schools of evolution show wide differences of opinion, the biometrician basing his method upon the doctrine of probabilities and proceeding quantitatively, the Mendelian basing his method upon the doctrine of unit characters and segregation and proceeding analytically and by distinction of qualities.
Which of these methods, if either, is psychology to adopt? It happens that psychology has already made a provisional choice; or rather, a choice has been made for her. Biometry has been predominantly concerned with human, Mendelism with non-human, inheritance. It is scarcely an accident, then, that biometrical methods were the first to exploit the mind of man. As you know, biometry's inspiration came from Francis Galton, traveler, explorer, geographer, anthropologist, student of evolution, psychology and sociology. The grandson of Erasmus Darwin, a representative therefore of one of the highly gifted strains of English blood, Galton has devoted himself to a quantitative study of the inheritance of talent and intellect, and to practical measures for purifying and improving the race. His interests revolve
