312 



Drs. E. F. Bashford and J. A. Murray. [Apr. 30, 



mouse, of the conclusion we advanced in ' Koy. Soc. Proc.,' vol. 73, January, 

 1904, and in the First and Second Scientific Keports of the Imperial Cancer 



Age. 



6—9 

 months. 



—12 

 months. 



—15 

 months. 



—18 

 months. 



—21 

 months. 



—24 

 months 

 and over. 



Total 



135 



110 



94 



21 



6 







3 



4 



r 



3 



2 







2 2 



3-5 



7-4 



14 -2 



33 -3 





Eesearch Fund, that, in animals as in man, the recorded frequency of cancer 

 varies with the opportunities for examining a large number of adult and aged 

 individuals. 



Account was taken of the age-incidence of cancer in the human subject in 

 the hypotheses of Thiersch* and of Cohnheim,f which were formulated for 

 man only, and are untenable to-day. The general biological significance of 

 the age-incidence of cancer, for which we have so often argued, has been 

 ignored, or, when mentioned, minimised by most pathologists, and, in recent 

 years, also by those engaged in the experimental study of the disease. It 

 is, perhaps, not too much to hope that the foregoing presentation of the 

 facts will henceforward impress on those engaged in the investigation of 

 cancer the urgent necessity for precise knowledge of the ages of men or 

 animals in whom the incidence of cancer is being studied. In particular, 

 the difference between mice 15 months old and 21 months old in their 

 liability to cancer at once invalidates completely all statements of the 

 relatively greater frequency of cancer in one group of mice than another, 

 when the exact age of the animals is not known. The same objection must be 

 raised to assertions of the occurrence of epidemics in other animals. Such 

 statements have been frequently made, and have received wide currency 

 since experiment demonstrated the possibility of the artificial transmission 

 of cancer from one animal to another of the same species, but only, however, 

 by implanting living cancer-cells, and also demonstrated that this form of 

 transmission could not be made responsible for the great frequency of 

 malignant disease. The above criticism therefore applies with destructive 

 force to all statements which have appeared up to the present on the occur- 

 rence of epidemics of cancer in mice and rats. Until it can be shown that 

 the conditions of experiment have altered the normal age-incidence of the 



* Thiersch, ' Der Epithelialkrebs, namentlich der Haut,' Leipzig, 1865. 

 + Cohnheim, 'Vorlesungen uber allgerneine Pathologie,' Berlin, 1877, 2nd Edition, 

 1882. 



