This is a study of fifty propositions from the Keller institution for the purpose of determining whether the type of material used in studies of the feebleminded has any bearing on the results. The author believes that higher grades of deficiency are more likely to be of hereditary origin than lower, so that studies made on defectives in which the subjects cover the whole range will give conflicting results. He reviews the literature covering the hereditary phases of the problem and cites various cases of his own of acquired oligophrenia. The study compares a high grade with a low grade group of defectives in regard to siblings and other relatives. Wildenskov concludes that there is a difference in the results of his observations in two groups significant enough so as to justify future studies on the subject of mental deficiency based on groups that are unified as to intelligence levels.