Relational Learning in Chimpanzees. Using the multi-pie-choice method, R. M. Yerkes (Compar. Psychol. Monographs, 10, No. 1; 1934) has studied in full detail the efforts of four young chimpanzees to solve a number of relational problems, such as choosing the right hand one of any set of compartments. He found that solution came by sudden discovery of the essential relation in about 50 per cent of the cases, and was fully retained in the control settings. Their ability to apply the correct solution to the control situations is markedly higher than that of other mammals, exclusive of other anthropoids and man, and their behaviour “obviously presages those expressions of human curiosity and originality which we call invention and discovery”. The affective condition of the apes is of the greatest importance in relation to success or non-success. There is a general discussion of different types of ‘learning’.