THE STUDY, OF CANCER 23 
passu with prolongation of growth (anaplasia), and, for 
apparently identical tumours, by assuming differences in the 
soil afforded by different individuals, and also by the several 
organs of any single individual. At the outset of our investi- 
gations, therefore, the growth of cancer under natural and 
experimental conditions was selected for study, as covering 
ground common to all the varied manifestations of tumours. 
The experimental study of the growth of cancer proved the 
permanence of the histological characters, and revealed that the 
cells possessed other qualities of a more or less permanent kind, 
escaping demonstration by other means. At the same time, 
experiment shed light on the relations subsisting, on the one 
hand, between the different properties of cells similar morpho- 
logically, and on the other hand the soil in which they are 
situated. Only by experiment has it become possible to study 
the growth of one tumour in different soils, and, what is equally 
important, the growth of different tumours in the same soil. In 
this way the varying clinical and histological features of the 
cancers of the mouse’s mamma, reproducing as they do in 
miniature all the difficulties encountered in man, have been 
relegated to subsidiary importance for the present, in favour of 
the study of the problem of cell assimilation, which, being 
common to all these tumours, permits us to unite in one general 
conception all the gradations and mutations exhibited in the 
growth of cancer. The experimental study of growth has 
already enabled us to compare tumours together as regards 
their biological behaviour (clinical behaviour). It has been 
proved that cancer grows better in young mice than in old; 
that some mice of the same age afford a more suitable soil than 
others ; that some sites of the body favour growth while others 
hinder it; that tumours seemingly identical (when examined by 
the aid of other methods) can be differentiated from one another, 
since they possess distinct properties by virtue of which they 
behave differently in the same soil; that one and the same 
tumour exhibits increased proliferation alternating with 
diminished proliferation in a similar nutritive environment, and, 
this being so, the fluctuations in proliferation are possibly 
expressions of an alternation in the assimilative powers of the 
cells of the tumours which exhibit them. Experimental pro- 
pagation further showed that proliferation can be maintained by 
normal bipolar division and that other forms of cell division are 
