THE STUDY OF CANCER 15 



rising to 90 per cent, at the third transference. When once the 

 initial difficulties have been overcome, the growth exhibited by 

 propagated cancer, as illustrated by several of the tumours we 

 have studied, soon passes beyond the bounds of all measure- 

 ment. A small piece of tissue weighing no more than o'oi grm. 

 frequently increases to i'j$ grm. of tissue in ten days. This is 

 an increase of over a hundredfold, and under suitable conditions 

 90 to 100 per cent, of the animals develop tumours through 

 many successive transplantations. This sequence of events is 

 not constant when tumours can be transplanted successfully. 



Some sporadic growths grow well from the outset of 

 propagation. For others a marked increase in the success 

 of transplantation is observed, frequently after three or four 

 transferences. For others again the percentage of successful 

 inoculations cannot be raised to, or maintained at, 100 per cent. ; 

 but remains constant within certain fluctuations. However, for 

 the majority of our carcinomata growth ceased at the third or 

 fourth transference. We have interpreted those differences to 

 signify primary differences in degree, inherent in the cells of 

 different tumours. They persist during continued propagation 

 in the majority of cases; so that each tumour exhibits from the 

 outset of propagation a constant behaviour within limits. We 

 have refrained from terming the increased success of trans- 

 plantation occurring in special cases " an increase of virulence," 

 as has been done by analogy with what occurs when bacteria 

 are propagated by the method of passage from one animal 

 to another. The term " virulence " should be dropped and 

 "adaptability" substituted, because the method of propagation 

 is essentially an artificial selection of the most rapidly growing 

 tumours, and their subdivision and distribution over a large 

 number of animals. Although the greater success sometimes 

 attending later inoculations suggests an alteration in the energy 

 and rate of growth of the tumour cells, the phenomenon referred 

 to may be explained without having recourse to this assump- 

 tion : (1) by an increase in the number of proliferating cells, 

 similar to what occurs in the propagation of domesticated 

 animals as in the following example ; and (2) by adaptation. 



The increase in the numbers of any form of fancy dog 

 which happens to become fashionable is due not to an artificial 

 shortening of the period of canine gestation, but to increased 

 breeding facilities, e.g. to multiplication of the number of those 



