Dr. Thorndike's careful measurements showed that it is impossible to draw a hard and fast line between identical twins and ordinary twins. There is no question as to the existence of the two kinds, but the ordinary twins may happen to be so nearly alike as to resemble identical twins. Accordingly, mere appearance is not a safe criterion of the identity of twins. His researches were published in the Archives of Philosophy, Psychology and Scientific Methods, No. 1, New York, 1905.

A First Study of the Inheritance of Vision and the Relative Influence of Heredity and Environment on Sight. By Amy Barrington and Karl Pearson. Eugenics Laboratory (London), Memoir Series V.