child became affected by glioma of the retina (a tumor originating in modified nerve cells of the eye). In another case a twenty-one-year-old man had 17 osteomata (tumors consisting of bone tissue) symmetrically arranged and his father had similar tumors. It is furthermore known that in certain cases polyps of the intestines are congenital and occur in several members of the same family. On the basis of such polyps cancer not infrequently develops. We have therefore reason to believe that heredity plays a role in a certain number of that type of cancers in which flaws in embryonic development are a factor.
In the case of the typical cancers of later life in the causation of which as we stated external stimuli play such a prominent part, it is very much more difficult to determine the significance of heredity. We know that the frequency of cancer varies very much in different races; but we have also seen that we can as yet not be certain how much this difference is due to factors inherent in the race (heredity) and how much it is due to variations in the mode of living and to preceding inflammatory conditions in the affected parts of the body. The ordinary methods of vital statistics which almost exclusively have so far been applied in cancer seem to show that in about 14—18 per cent, of persons affected with cancer other cases of cancer occurred in the family. Now it is doubtful whether this incidence is greater than should be expected according to the law of probabilities. Even extensive statistical studies of this character can evidently not solve the problem. We must rather turn to intensive studies of the incidence of cancer in various families for a solution. There are indeed already some data available which seem to indicate the existence of a hereditary factor also in the causation of the typical cancers of more advanced age. In certain families, as for instance one reported by Broca, the incidence of cancer has been extraordinarily high. A. C. Garmann found that in a certain district of Norway, the population could approximately be divided into 20 distinct families, and 72.8 per cent, of all cases of cancer occurred in a single one of these 20 families. J. Levin has begun to use the statistical material collected at the Eugenics Eecord Office of the Carnegie Institution in Cold Spring Harbor for such intensive statistical studies. In one family on which he has reported recently he found, that a fraternity in which one or more members suffer from cancer, usually shows in a previous generation a cancerous member either on the paternal or maternal side or on both sides. It may be expected that a continuation of such studies will decide definitely the problem as to the significance of heredity in human cancer. As we shall see later, in the case of animal cancer the great importance of heredity has recently been established.
Especially since the discoveries of various microorganisms as the cause of certain diseases, the possibility was always in the minds of investigators, that also in cancer besides the conditions enumerated micro-
