THE STUDY OF CANCER u 



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 infinitum, 

 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, 



