THE INHERITANCE OF DISEASE 5O3 



in the resistance. Calculation of the results with the guinea 

 pigs has shown that such factors as age, abihty to gain 

 weight on a mixed diet, absolute weight, etc., factors in 

 part determined by the inheritance and in part by environ- 

 mental conditions, can account for something less than lo 

 per cent of the observed variation. Factors of direct influences 

 and directly dependent on the inheritance account for 

 somewhat over 30 per cent of the variation. There remain 50 

 or 60 per cent of the observed differences between individuals 

 at present not accounted for. Such factors as differences in 

 kind and amount of food consumed, "accidents" incident 

 to the spread of the disease within the animal (implantation 

 in particular organs, etc.) are doubtless to be included in 

 this category. Essentially it is to be counted as an accompKsh- 

 ment that we may at present be quite certain that inheritance 

 does play a recognizable part in the prevalence of an infectious 

 disease. 



CANCER AND OTHER MALIGNANT TUMORS 



In general the state of our knowledge of the factors 

 underlying the occurrence of malignant tumors is not 

 dissimilar to that with regard to tuberculosis. The evidence 

 from human sources is of about the same order but less 

 significant on the whole. Tumors have been alleged to 

 frequent occasionally certain families while others remain 

 quite untouched. In the mass there is the sporadic, occasional 

 appearance of a tumor case in most family histories. Cancer 

 is not believed by most authorities to be an infectious 

 disease although the fact that it can apparently be initiated 

 in man and animals by chronic irritation with various 

 substances, even by various parasites, creates many resem- 

 blances between tumors and infections. If the tumors 

 classified as sarcomata are included there are cases in which 

 the utmost consideration of detail fails to reveal any precise 

 reason why they should not be accepted as infections; 

 and yet because of that fact that these appearances suggesting 

 infection are the exception rather than the rule, most 

 scientists hold in reserve the thought that even in these cases 

 it is more than possible that some other explanation will 



