A very serious handicap is met in the effort to make known the facts of heredity and to place these facts in their proper relation to other biologic data, for the purpose of establishing the exact value of heredity among the biologic influences, so that we may have a firm basis for further departure, and not be forced continually to replow the same ground. This is particularly true in attempting to establish the relation of heredity to disease. This serious handicap consists in what may well be called fundamentalism in the scientific field; that is, preconceived ideas and prejudices, not based on scientific research, but instead on general impression, hearsay evidence and the like, handed on from one to another and thus established in general opinion. Scientific terms, frequently misunderstood and misused by clinicians and research workers, form a strong background of entrenchment to prejudice and preconceived opinion. One of