372 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



Before attempting to evaluate the results of these investigations, we may 

 consider some earlier findings of a related nature. The study of multiple 

 spontaneous tumors developing in the same individual was begun as early as 

 1907, when the writer, working with mice, noted that the structure of multiple 

 carcinomata originating in the same animal was very similar, although not 

 identical. At that time we suggested that it might be possible through trans- 

 plantation of such tumors to determine whether the characteristic behavior 

 of different carcinomata in strange hosts was due to factors inherent in the 

 tumors or in the hosts. Woglom (1919), who had carried out such transplanta- 

 tions, found that the large majority of multiple tumors arising spontaneously 

 in the same animal, behaved similarly after transplantation into the same 

 strain of mice, but in a minority of cases differences did occur. Especially 

 striking in this respect was the transplantation of three spontaneous tumors 

 which had developed in the same mouse. One of these was readily trans- 

 plantable into other mice, while the other two retrogressed following a tem- 

 porary period of growth, and one tumor retrogressed more readily than the 

 other. However, in these experiments Woglom wished to determine whether 

 the behavior of tumors after transplantation depended upon adaptation of the 

 tumors to the environment as it existed in the animal in which they had 

 originated, or whether it depended upon the growth energy of the tumor 

 at the time of transplantation. In the former case all the transplants should 

 behave in a similar manner, since all these tumor cells had been reared in 

 the same environment, while in the latter case the tumors should behave 

 differently from one another because the growth energy is a variable factor, 

 which, according to Bashford, differs at different times even in the same tumor. 



The basic assumption underlying the interpretation of Strong and his col- 

 laborators is that the difference in the behavior of two tumors arising spon- 

 taneously in the same mouse is due to differences in the mutations of genes 

 in somatic cells and, therefore, to the differences in the gene sets of these 

 two tumors resulting from these mutations. But this, it seems, is not the 

 only possible interpretation of this finding. We know that various normal 

 and also embryonal tissues show different degrees of transplantability ; thus, 

 cartilage may be homoiotransplanted successfully in cases in which thyroid 

 cannot, although both tissues can be autotransplanted equally well. These two 

 tissues, when taken from the same individual, possess the same individuality 

 differential but differ in the constitution of their organ and tissue differentials, 

 and this latter difference may cause variations in their sensitiveness and trans- 

 plantability. We also know that normal tissues differ much in the growth 

 momentum which they possess ; for instance, the normal and sensitized uterine 

 mucosa may exhibit quite a different degree of proliferative activity after 

 homoiotransplantation. Now, if we assume that during the transformation 

 of normal tissues into cancerous tissues a graded increase in growth energy 

 occurs and a concomitant change takes place also in the resistance to the 

 injurious effects of transplantation, and if we furthermore assume that in 

 two tumors, developing spontaneously in the same individual, this transfor- 

 mation has progressed to a different degree, then we could explain the ob- 



