26 SCIENCE PROGRESS 
affected with cancer. This increase in the amount of physio- 
logically active hydrochloric acid is to be regarded as a com- 
pensatory response to the needs of a normal mouse plus a 
tumour. No assumption is made in inferring that the increase 
in hydrochloric acid is required for the digestion of proteids. 
The increase in hydrochloric acid is in all probability accom- 
panied by increased secretion of pepsin and trypsin, the ferments 
concerned in proteid digestion, although this is difficult to 
determine in mice. The increased activity of proteid digestion 
is easily comprehended. Whereas, in a general way, the com- 
bustion of carbohydrates suffices for the output of energy in 
an animal organism, proteids are essential to the building up 
of protoplasm, and the building up of protoplasm is proceeding 
apace in the continual production of new tumour cells. 
On a preceding page I have pointed out the specific characters 
of the proteids which even nearly allied animals reconstruct from 
identical pabulum, and how the mouse alone supplies the nutritive 
environment or pabulum suited to the continued growth of 
mouse cancer. These facts are to be remembered in the present 
connection, for there is no need to assume that the increase in 
proteid digestion involves any other divergence from the normal 
process. No doubt proteid digestion prepares the food-stuffs 
for absorption into the fluids of the body in the usual way, 
and for their utilisation by its constituent cells and by the cells 
of the tumour. So fine an adjustment can only mean that the 
cancer cells of the mouse and normal mouse cells compete for 
the products of proteid digestion. In view of what we know 
of the specificity of these processes of nutrition, on which so 
much light has been thrown by Starling and Bayliss, this 
implies that the cells of mouse cancer are mouse cells able to 
impress their needs for food on the mouse as a whole by their 
higher assimilative energy. The facts reveal, at any rate, how 
great are the demands for food which growing tumours make 
on healthy and young adult animals, and at the same time 
show that the normal animal responds to these demands at 
least by increased proteid digestion. These observations are 
the mere beginnings of the experimental study of the problems 
of metabolism in cancer, and which are not easy to attack in 
so small an animal as a mouse, and will be attacked still more 
profitably in the case of larger animals, eg. the dog. The 
problems of excretion which may have practical importance 
