cause, twins should show greater resemblance in the case of traits much subject to training, such as ability in addition or multiplication, than in traits less subject to training, such as quickness in marking off the A's on a sheet of printed capitals, or in writing the opposites of words."
The data were elaborately analyzed from many points of view. They showed (1) that the twins 12-14 years old were not any more alike than the twins 9-11 years old, although they ought to have been, if environment has great power to mold the character during these so-called "plastic years of childhood." They showed (2) that the resemblance between twins was two or three times as great as between ordinary children of the same age and sex, brought up under similar environment. There seems to be no reason, except heredity, why twins should be more alike. The data showed (3) that the twins were no more alike in traits subject to much training than in traits subject to little or no training. Their achievement in these traits was determined by their heredity; training did not measurably alter these hereditary potentialities.
"The facts," Professor Thorndike wrote, "are easily, simply and completely explained by one simple hypothesis; namely, that the nature of the germ-cells—the conditions of conception—cause whatever similarities and differences exist in the original natures of men, that these conditions influence mind and body equally, and that in life the differences in modification of mind and body produced by such differences as obtain between the environments of present-day New York City public school children are slight."
"The inferences," he says, "with respect to the enormous importance of original nature in determining the behavior and achievements of any man in comparison with his fellows of the same period of civilization and conditions of life are obvious. All theories of human life must accept as a first principle the fact that human beings at birth differ enormously in mental capacities and that these differences are largely due to similar