THE: STUDY: OF “CANCER Il 
examined. That cancer develops with increasing frequency 
with advancing years in all animals, whether their lives be 
long as in the case of man, or limited to two or three years 
as in the case of the mouse, is a biological law of universal 
application. The only class in which cancer has not yet been 
found, viz. the reptilia, is the exception proving the rule, for 
in them life may extend to hundreds of years, and the oppor- 
tunity for examining a sufficiently large number of aged reptiles 
does not exist. The same applies, if in lesser degree, to the 
apparent rarity of the disease in wild animals generally, since 
they do not naturally survive the functional activity of their 
teeth or their reproductive system. 
The greater recorded frequency of cancer in domesticated 
mammals as compared with wild animals implies, not that the 
disease is communicated from man, but simply, that man pro- 
tects them and provides for them, so as to permit of their 
attaining their respective cancer ages. 
In the case of the organs of the human body, cancer exhibits 
a corresponding dependence on the duration of life, e.g. chorion 
epithelioma develops before birth and presents a perfect parallel 
to the incidence of cancer in short-lived animals. In a general 
way it may be said, since space prevents my discussing details, 
other organs present all gradations up to the skin, which is 
functional so long as life lasts, and in which the maximum 
incidence of cancer is only attained in extreme old age. The 
age of the individual organism is of less moment than the 
senescence of its several tissues in determining the incidence 
of cancer. Hence cancer may occur at any age, in the new-born, 
and even before birth. As a matter of fact it is more frequent 
at birth than between the tenth and fifteenth years of life. It 
is therefore wrong to assert that cancer is caused by “old age.” 
The generalisation as to the constant association of the 
incidence of cancer with senescence adds greatly to the signifi- 
cance of its unbounded proliferation, when propagated by 
transference from one set of mice to another ad infimitum, 
since the size of the body and of its individual organs, as well 
as the length of life, are specific for each species of animal. 
They distinguish different species as sharply as any of the other 
criteria on which the systematic zoologist depends for his 
classification, e.g. as sharply between the mouse and the rat as 
between the mouse and the elephant. As I shall describe below, 
