340 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



extended through many more generations than previous ones, transplanta- 

 tion was used as a method for analyzing the characteristics of the tumor 

 cells and the interaction between tumors and hosts; there was thus initiated 

 the subsequent large number of investigations into the biology and causes 

 of cancer, which has continued with increasing intensity until the present 

 day and which has contributed much to the solution of these problems. 



The objective of the writer was the study of the characteristics of tumor 

 cells, of the factors which made them behave in their own peculiar way, of 

 the possibility of separating a living agent responsible for the tumor growth 

 from the transplanted cells ; in addition, there was the analysis of the causes 

 and mechanism of the transformation of normal into tumor cells, and, above 

 all, the comparison between the behavior of transplanted normal and can- 

 cerous cells, which made possible a critical examination of what we now call 

 the organismal differentials of tumors. Jensen approached these investiga- 

 tions primarily from the point of view of bacteriology and immunology, his 

 central problem being the possibility of obtaining an active and passive im- 

 munity against tumor growth, similar to that which can be obtained against 

 bacteria and their toxins. In a similar way, the subsequent investigations of 

 Ehrlich and Apolant, Gaylord and Clowes, Bashford and Murray and their 

 collaborators, and many other well known workers, were largely concerned 

 with the problem of immunity, but gradually these studies have led back 

 again to a comparison between the behavior of normal and tumor tissues, 

 since it became more and more evident that some of the most important 

 characteristics of tumor cells are shared with normal cells. Thus, in the end 

 both these series of investigations contributed also to the analysis of or- 

 ganismal differentials in general. 



We shall now compare the various types of transplantation of tumors 

 with those of normal tissues and determine wherein they resemble each other 

 and wherein they differ. 



Auto- and Hotnoio trans plantation of Tumors 



We have seen that normal tissues behave very differently after auto- and 

 after homoiotransplantation. In the former, the individuality differentials 

 of host and transplant are identical, while in the latter they are different. 

 One of the marked differences between normal tissues and tumors consists 

 in the fact that some tumors can be homoiotransplanted from generation to 

 generation into a percentage of animals of the same species, which varies in 

 the case of different tumors, whereas, such a serial homoiogenous trans- 

 plantation does not succeed with normal tissues. But this is not true of the 

 majority of tumors ; while there are some which can be readily transplanted 

 into animals belonging to the same species, irrespective of family or strain, 

 the large majority grow only in animals belonging to the same closely inbred 

 strain, and very much less or not at all in other strains; again, others grow 

 in a certain percentage of mice from mixed strains of the same country in 

 which they had developed, but do not grow in strains bred in distant coun- 

 tries. To cite an example: a carcinoma of the mammary gland which had 



