16 SCIENCE PROGRESS 



who find it profitable to breed that kind of dog. There is a 

 multiplication of the number of dogs breeding, so in the artificial 

 propagation of cancer we multiply the number of cells pro- 

 liferating. I discuss later the possibility of biological alterations 

 leading to increased rapidity of cell growth. Propagation segre- 

 gates in the first instance the cells which can accommodate 

 themselves to the artificial conditions, and then provides for 

 their nutrition and proliferation. Thus there are more of the 

 suitable cells in each of the later grafts, and within a given 

 period, this primary advantage gives bigger tumours from each 

 single graft, and a higher percentage of tumours over the total 

 number of animals inoculated. 



A violent change in environment is effected when a tumour 

 is removed from the animal in which it developed. The manner 

 in which the change is effected is important. Whereas we have 

 succeeded in transplanting 66 per cent, of the sporadic growths 

 we have discovered, Ehrlich has transplanted 14 per cent, of his 

 tumours successfully. The discrepancy is certainly due to the 

 difference in method. As a rule only the tumours best suited to 

 propagation survive Ehrlich's procedure, while we have obtained 

 a broader basis for the study of the growth of tumours, since 

 our material includes tumours of all degrees of transplantability. 

 One group of tumours does not survive the process at all. A 

 second group grows for a variable time and then dies, being 

 unable to adapt themselves to a strange and presumably an 

 unfavourable environment. The tumours of a third group 

 are able to adapt themselves gradually. The fourth group 

 adapt themselves quickly or at once, and this is the small 

 group which are easily propagated. It is possible that among 

 them there are tumours surviving transplantation, not only 

 because of the inherent properties of their cells, and their 

 capacity for adaptation, but because the change in environ- 

 ment is in reality to a more favourable soil. In the third and 

 fourth groups there must be also more and more cells suited to 

 propagation in the grafts at each additional transference, till a 

 maximum is reached. Although the group of tumours giving 

 maximal success on propagation are of practical importance in 

 experiment, because of the ease with which they permit of 

 controls to attempts to modify growth, their theoretical value 

 depends on their relation to the varied degrees of energy of 

 growth exhibited by the whole group of tumours. It is 





