36 TEE POPULAR SCIENCE MONTHLY 



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 in- 

 flammatory 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 extra- 

 ordinarily 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 Insti- 

 tution 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 mater- 

 nal 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 in- 

 vestigators, that also in cancer besides the conditions enumerated micro- 



