6 Trans. Acad. Sci. of St. Louis 



ferent strains; in some families almost 100% of all fe- 

 males become cancerous; in other families almost none 

 become diseased. This constitutional characteristic is 

 transmitted by heredity from generation to generation, 

 with only slight modifications. Even the age at which 

 the cancer appears in mice is determined by heredity. 

 In other kinds of animal cancer, heredity plays a similar 

 role. And there is much evidence that the same holds 

 good in the case of man. But in man conditions are 

 much more complicated through continued interbreed- 

 ing. This prevents the hereditarily transmitted consti- 

 tutional factor from being as clearly recognizable as in 

 the case of animal cancer where it can be followed in 

 isolated families. But even in man indications are not 

 lacking that heredity plays the same part as in animal 

 cancer. This is especially clear whenever interbreeding 

 has been less prominent; and the recent studies of C. C. 

 Little, of the Eugenics Record Office of the Carnegie 

 Institution for the Study of Evolution, show that an 

 hereditary factor can be shown to exist in human cancer 

 generally. 



As we stated above, the heredity factor is usually ac- 

 companied by internal or external stimuli; and these 

 stimuli must cooperate with the hereditary disposition 

 which in certain cases seems to imply a tendency to a 

 special kind of cancer, rather than a general tendency to 

 cancer. These stimuli transform the potential heredi- 

 tary disposition into an actual cancer. These stimuli 

 may be chemical or mechanical. The effect exerted by 

 the ovary, to which we referred above, is an example of 

 a chemical stimulus. Certain parasitic worms in the 

 body of the host may likewise stimulate some tissues to 

 <5ancerous proliferation. This is probably also due to 

 a chemical stimulus emanating from the parasite. LonL' 

 continued irritation functions as a mechanical stimulus. 

 Roentgen rays acting on unprotected skin through a long 

 period of time may also cause a mechanical irritation. 



