12 SCIENCE PROGRESS 



cancer frequently stops growing, and this is the most encouraging 

 result obtained by experiment. Nevertheless, there is as yet 

 little reason to suppose that limits corresponding to those set 

 to the growth of organisms and their organs, are necessarily 

 set to the growth of cancer in any of the vertebrates, except 

 when artificially set by the death of the animal in which it 

 originates or is growing. The primary transplantations of 

 our sporadic tumours have succeeded for 66 per cent., and 

 ultimately all may be found to be transplantable. Hence, since 

 a mouse tumour has now been growing twice as long as a 

 mouse lives, the more limited evidence of a similar phenomenon 

 in other vertebrates, e.g. rats and dogs, is probably due mainly 

 to insufficient experimentation, and to failure to supply suit- 

 able soil. On the one hand, we have the limited powers of 

 proliferation of normal tissues which never produce tumours 

 when transplanted, and the unlimited proliferation of cancerous 

 tissue ; on the other hand, we have the dependence of the 

 inception of cancerous change on senescence of the tissues 

 as determined by the laws limiting the duration of life 

 specifically. We have stated that in this association lies the 

 crux of the problem of cancer, and R. Hertwig in referring 

 to physiological senescence, and others, have also noted its 

 biological importance. This association involves a problem 

 which cannot be attacked directly at present, because we are 

 still unable to determine the inception of cancer experimentally. 

 We can, however, approach it indirectly by the prolonged study 

 of the growth of cancer when once it has started, and endeavour 

 to determine if it be purely vegetative, dependent entirely on 

 the supply of adequate and suitable food, or be perhaps also 

 maintained and renewed by some intra-cellular reorganisation 

 occurring periodically, at a time when growth tends to cease 

 naturally. In short, we must find out if the study of growth 

 gives us any clue to its nature and origin. We can also study 

 the conditions favouring and retarding its continuation or modi- 

 fying it. Thus, the experimental study of cancer is at present 

 essentially the experimental study of the growth of cancer ; 

 but this is no subsidiary attribute — it is in reality that property 

 of cancer by virtue of which it destroys human life. It is 

 customary to consider the growth of tumours as expansive or 

 " benign," and infiltrative or " malignant," a classification corre- 

 sponding also to their clinical behaviour. As a matter of fact 



