THEORIES AND PROBLEMS OF CANCER 119 



The cells forming the tumours produced after a long suc- 

 cession of graftings must possess some characters that were 

 not at all necessary to those forming the primary growth ; 

 they are able to resist a strange environment and the reaction 

 on the part of the cells of the host which does not exist at 

 all or only in a very slight degree in the case of a primary 

 cancer ; they go on multiplying during periods several times 

 as long as the period of life normal in the species of 

 animal in which the primary tumour originated from which 

 they were obtained ; and the cells produced after a number 

 of sojourns in strange hosts, involving a number of cell 

 generations many times greater than could possibly have 

 occurred had they remained in the original host, sometimes 

 exhibit very striking and obvious morphological differences 

 from the cells of the original primary tumour. In this con- 

 nexion it is well to bear in mind the facts relating to the 

 general potentiality of differentiation retained by the cells of 

 the soma. 



It seems probable, from a theoretical point of view, that 

 the form of selection to which the cells are subjected in strains 

 of transmissible tumours must tend to preserve those in which 

 the potentiality for independent existence is greatest : that 

 the greater the number of cell generations produced away 

 from the environment in which the ancestral malignant cells 

 arose and the more numerous the different environments 

 through which the descendants have passed, the more similar 

 their characteristics should be to independent organisms. This 

 theoretical probability seems to be borne out by observed facts. 



