10 SCIENCE PROGRESS 
origin of cancer, it is unnecessary for growth to continue. 
(3) The conditions of the origin of cancer were to be sharply 
distinguished from those of continued growth. (4) The pro- 
gressive, apparently vegetative, growth was to be explained 
as inherent in the cancer cells without the assumption of the 
stimulus of extraneous agents. The amount of growth pre- 
sented a cell problem of importance in itself and required to 
be further analysed. (5) The purely cell problems of cancer 
required to be attacked by the methods of experimental and 
comparative biology rather than from the narrower standpoint 
of human pathology. 
Those conclusions would have been illegitimate if drawn 
only from experiment at that time. They were advanced 
also on the basis of a comparative study of the disease in the 
vertebrates down to marine teleostean fishes living in a state 
of nature, and after careful statistical studies of the national 
mortality from cancer, and of its occurrence in the patients of 
London hospitals. The zoological distribution of cancer made 
it obvious that it developed in man, independently of any direct 
influence proceeding from civilisation, diet, or other one of many 
inconstant external factors. Subsequent studies in the same 
direction, and also into the ethnological distribution of the 
disease, have but rendered this inference more justified. The 
disease has been found in all those races of mankind among 
whom diligent search has been made. Where it was said 
to be rare it has been found to be common—e.g. in Japan 
some 25,000 deaths are annually recorded from cancer. Where 
it was said to be absent it certainly occurs. Of course, no 
comparison or statement of relative frequency is possible 
between civilised and aboriginal man, and between tame and 
wild animals. The fact of prime importance is that the disease 
is of universal occurrence, pervading all forms of vertebrate 
life, and everywhere adapting itself to the conditions presented 
by different species. The disease is universal, but the difficulty 
of transferring it even to individuals of the same species is 
unique, and when successful is not to be confounded with 
the production of cancer ab initio. 
Throughout the entire distribution of the disease two facts 
stand out prominently: The number of cases recorded is in 
direct proportion to the care with which it is possible to make 
examinations, and to the number of adult and aged individuals 
