390 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



bility of tumors and complicate the analysis of the organismal differentials. 

 However, it is necessary to distinguish from real adaptive processes a condi- 

 tion which may lead perhaps to similar changes, but is different. As a result 

 of selective transplantation, lines of tumors, differing in certain character- 

 istics, may be separated from the original tumor. By always selecting the 

 most actively growing tumors for transplantation it was thought possible 

 to separate from the tumors with ordinary growth energy and transplanta- 

 bility, a line which exceeded this average tendency. In these instances we 

 would have to deal not with adaptive changes in the tumor — the character- 

 istics of the tumor cells remaining the same throughout — but with a selection 

 of certain types among several already in existence. In the case of true 

 adaptation, on the other hand, actual changes in the characteristics of tumor 

 cells would occur. Ehrlich used such a method of selection in order to obtain 

 readily transplantable tumors. He compared this procedure with that em- 

 ployed in order to increase the virulence of bacteria, where, in serial inocu- 

 lations of certain microorganisms into susceptible animals, the most virulent 

 strain of bacteria was selected for each inoculation. However, Ehrlich 

 believes that at the same time changes take place in the tumor cells in the 

 course of transplantation. 



In contradistinction to the increase in growth energy in successive genera- 

 tions of transplantations which we had observed, Ehrlich stressed the increase 

 in percentage of takes in successive generations of transplanted tumors, and 

 in accordance with his conception of athrepsia, as the main factor which 

 determines the life and growth of cells, he explained the increase in trans- 

 plantability in the course of serial transplantations as due to a new produc- 

 tion of "nutriceptors" in tumor cells, which, according to his views, took 

 place under the unfavorable conditions following transplantations into new 

 hosts. Thus, the behavior of tumor cells was explained in the same way as 

 the origin of strains of trypanosomes resistant to trypanicidal substances. 

 Ehrlich operated therefore, essentially with one variable factor, namely, the 

 difference in the ability of different cells to attract foodstuffs to themselves, 

 and he assumed that a selection takes place in cells which differ in their 

 power to respond to unfavorable conditions with changes in their nutriceptor 

 apparatus. However, it would be difficult to explain on this basis the fact 

 that the variations in growth energy and transplantability which do occur 

 do not always take a parallel course ; Ehrlich did not take into account the 

 differences which exist in the individuality differentials of different hosts, and 

 he also failed to consider the effects of separating strains of hosts. Further- 

 more, he did not consider the relations which exist between growth energy 

 and transplantability. 



Various observations make it very probable that adaptive changes in the 

 constitution of tumors can actually take place. The considerable increase in 

 the number of takes, which has been noted by different investigators in the 

 course of serial transplantation of spontaneous tumors into strains of animals 

 in which they at first grew only with difficulty, is probably at least partly due 

 to certain adaptive changes which have arisen in the tumors in the new host. 



