THE:SLEUDY. OF CANCER 9 
naturally from one individual to another, equally extensive 
observations have been necessary. As I shall show later, the 
cancer cell of the mouse is in all probability a mouse cell 
incapable of nourishing itself and proliferating, except when 
in intimate organic union with the body, and nutrient supply, 
of the living mouse. Ehrlich has shown that a transitory growth 
follows the transplantation of one of his mouse tumours into 
rats. We found that the same power is possessed by other 
mouse tumours. Growth, however, speedily ceases in rats, 
although it is retained unimpaired if the cells be re-transferred 
to mice after a short sojourn of six to eight days in the rat’s 
body. A mouse tumour does not grow equally well in all mice. 
We found difficulty at first in getting Jensen’s Danish tumour 
to grow in English mice, and Michaelis and others have entirely 
failed to get it to grow in the mice of Germany, England, and 
other countries. 
The progress made by the demonstration, that cancer could 
be transplanted and artificially propagated ad libitum, was not 
of the obvious kind appealing to the multitude. If transference 
had proved that the tissues of the new animal acquired cancerous 
properties, ze. that the disease could be conveyed by way of 
infection, our knowledge of its nature would have been advanced 
enormously at one step. Then investigation could have been 
legitimately limited at once to identification of the agent and 
of the channels of infection. 
The nature of the transference of cancer was out of accord 
with all known processes of infection, in this respect merely 
presenting old problems in a new light, as well as new problems 
for solution. However, Murray and myself were able to draw 
certain conclusions other than those drawn by Jensen, which 
have been abundantly confirmed since by others: (1), The 
amount of proliferation exhibited is enormous, once the primary 
difficulty of transplanting a mouse tumour at all has been 
overcome. The proliferation is then out of all conformity with 
the laws of growth in vertebrate organisms. The cancerous 
tissue retains its histological characters. (2) If a large number 
of healthy mice be considered, growth proceeds readily in mice 
in which cancer is rare naturally. Although the maximum 
incidence of cancer occurs in the later years of life, growth 
proceeds as well, and even better, in young animals than in 
old. Therefore, if senescence is intimately bound up with the 
