18 SCIENCE PROGRESS 



natural variations, an increase in its rate may be one result 

 of our artificial selection in long-continued propagation. In 

 passing I may merely mention it is now well established that 

 the amount of proliferation can be influenced in the opposite 

 direction — viz. diminished by exposure of a tumour to heat. 

 The environment may be of great importance when a tumour 

 is propagated for years. Theoretically the long-continued 

 growth of cancer cells in the soil provided by the mice of one 

 stock or country may handicap them for growth in the different 

 soil provided by mice of another; but, whether this be so or 

 not, augmented adaptation to an accustomed soil may result 

 in an enormously increased proliferation. In this direction 

 therefore the cells of later generations may be biologically 

 very different from those remotely antecedent. What occurs 

 when a tumour is propagated by transference through many 

 generations of mice is determined not so much by the number 

 of transferences, as by the long duration of the particular 

 environment, breeding as it were a cell of particular quality. 

 If one follow the process of artificial propagation back step 

 by step to the primary animal, there is no reason why one 

 should stop there, since the gulf between the growth of 

 cancerous tissue and normal tissue after transplantation has 

 been bridged over. What takes place in artificial propagation 

 may well be but an artificial reproduction of what had long 

 been going on naturally in the animal in which cancer 

 developed. However, it is inadvisable to pursue such specula- 

 tions further at present, for the conditions of the growth of 

 cancer in the spontaneously attacked may be different from 

 those in normal animals, although the lesions are the same. At 

 any rate, the differences between individuals may be of prime 

 importance in determining the nature of growth in the primary 

 transplantation as well as in the individual spontaneously 

 attacked. It seems, at least, that the preceding considerations 

 give us a deeper insight into the nature and clinical behaviour 

 of the cancers of man during their continued growth, and when 

 they exhibit what appears to be a change to a more malignant 

 condition. 



The phenomena of growth can all be explained without the 

 assumption of microbic interference. And this brings me to the 

 further consideration of some matters of clinical importance. 

 When the growth of a tumour under artificial propagation is 



