336 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



instance represents the stimulating factor, is present mainly in the female 

 sex. It has furthermore been observed in this kind of cancer that in hybrids 

 the mother strain is much more potent than the father strain in determining 

 the cancer rate, and this is due to the factor transmitted with the milk from 

 the nursing mother to the offspring. An inverse relation seems to exist be- 

 tween the intensity of stimulating factors and the strength of the genetic 

 factors needed for the transformation of normal tissues into cancer. Either 

 of these two sets of factors alone may be effective in certain cases, if it 

 reaches a given intensity. This relation can be expressed by the equation: 

 H (hereditary constitution) X S (stimulation) = C (cancer). 



(3) In the main, three kinds of viruses or virus-like substances are known 

 as causative factors in cancer, namely, (a) the virus of cottontail rabbits, 

 which may give origin to papilloma, but under certain conditions, also to 

 epidermal cancer. It probably acts as a stimulating factor, comparable to 

 the other stimulating factors already mentioned, (b) The milk factor, which 

 participates in the origin of mammary gland carcinoma in mice and acts in 

 association with hormonal and genetic factors. It may have also a slight 

 effect in mouse leukemia. Its mode of action is not definitely known, but 

 there is some indication that it also may act as a sensitizing factor, (c) In 

 avian sarcomas, agents can be separated from the tumor cells by filtration 

 and certain other means. These agents seem to cause this type of cancer 

 directly, without inducing first a preparatory growth period. They are largely 

 but not entirely species- and organ-specific. They represent a carcinogenic 

 substance in the strict sense, while the other factors apparently are growth 

 promoters. A similar agent is, perhaps, present in renal carcinoma of the 

 frog. 



It is believed by some investigators that somatic mutations in tissue cells 

 may be responsible for the origin of cancer; however, there are a number 

 of facts which make this interpretation very improbable. On the other hand, 

 mutations which affect the germ cells may determine the degree of hereditary 

 tendency to the development of a certain type of cancer in individuals, strains 

 or species. Three theories concerning the origin of cancer are under con- 

 sideration at the present time : ( 1 ) The somatic mutation theory assumes 

 that all the other factors mentioned cause cancer by inducing changes in the 

 genes of a certain cell, which then becomes cancerous and gives origin to 

 the other cancer cells. There are very serious objections to this theory, which 

 is, therefore, in all probability not correct. (2) The virus theory assumes 

 that viruses are the essential causes of all cancers, and that all other factors 

 are effective only if they make it possible for viruses to invade cells and to 

 activate their growth. While the possibility of such a function of viruses is 

 indicated, especially in the case of avian sarcoma, there are some difficulties 

 also to the application of this theory in many other cancerous states. (3) 

 The theory that step-by-step increases in growth momentum of tissues lead 

 to intermediate stages of sensitization, and ultimately to irreversible, can- 

 cerous proliferation, perhaps through the mediation of an autokatalytic 

 growth substance. This seems at present the most likely general explanation 



