504 HUMAN BIOLOGY 



be found, that eventually it will appear that all the true 

 malignant tumors (including most forms now classed as 

 such) will be found to originate in causes resident within 

 the body. 



Tumors bear a certain resemblance to infection in that 

 those which originate in animals are often transferable to 

 other animals of the same species by a succession of trans- 

 plantations of the tumor tissue, or in some instances by 

 extracts of this tissue containing no intact body cells. The 

 conditions governing the transplantation are such as to 

 make the influence of inheritance very apparent. These 

 tumors are never transferable outside the species of animal 

 in which they originate. For instance, mouse tumors can only 

 be propagated in mice. Within the species they are transfer- 

 able with great difficulty when at all, from one race to 

 another. It is quite likely that this line is as rigid as the species 

 line, but it is impossible to be sure because in the domesticated 

 mice, rats, and fowls which are available for experimentation, 

 racial lines have been hopelessly confused by repeated 

 intercrossings. However this may be, it has been the common 

 experience that when transplants of a spontaneous tumor 

 are attempted they succeed in but a small percentage of 

 the subjects unless by chance the subjects are the immediate 

 relatives of the animal bearing the original tumor, when the 

 percentage of success may be, and often is, greater. It is 

 evident that a racial and familial variation in the suscepti- 

 bility-resislance ratio is operative in the tumor transplanta- 

 tion experiments. 



This variation in resistance has been the subject of 

 thorough genetic experimentation and analysis in certain 

 instances. When the Japanese waltzing mouse and the 

 common tame mouse were compared it was found that their 

 differences with respect to tumor transplantability across the 

 race line must be under the influence of at least twelve 

 separately inherited unit characters. The reasoning applied 

 to the case of tuberculosis in the preceding paragraphs 

 holds here. We should expect the familial evidence for 

 inheritability in the human race to appear only very occa- 

 sionally. Even less is known about the fundamental nature of 

 the inherited characters in tumors than in tuberculosis. 



