502 HUMAN BIOLOGY 



Now for the color inheritance in these same families of 

 guinea pig, Wright has made out the operation of at least 

 seven separate characters and he calculates that the possible 

 recombinations of these would lead in this stock alone to no 

 less than 25,000 color varieties of guinea pig. There is, as 

 suggested previously, no apparent reason for assuming that 

 tuberculosis resistance is determined (in so far as it is depend- 

 ent on inheritance) in any other or more simple way than this 

 in either guinea pigs or humans, and we are led to beheve 

 that the possible varieties of humans from the point of view 

 of their behavior with respect to tuberculosis must actually 

 number in the thousands. In fact it appears rather remark- 

 able on this bas'S that famihal characters are ever recogniz- 

 able even though the assumption of an inheritable influence 

 were uncontested. One is incHned to think that there must be 

 favored associations of characters which divert the results 

 into fairly well-defmed main channels in many cases. How- 

 ever this may be, and allowing all possible latitude for 

 famihal association of inheritable quahties, it is plain that 

 under the systematic outcrossing which is the rule in human 

 matings, the observed fact that, taken by and large, the 

 individual variations in resistance to tuberculosis are more in 

 evidence than the family hkenesses is what one would expect. 



In no other infectious disease of man has it been made so 

 evident that inherited quahties are influential in either the 

 prevalence or character of the disease in the individual. 

 Instances are reported among animals, both in reference 

 to spontaneous epidemics and inoculation diseases where 

 the result is as definite or more so, and where the inheritance 

 of the controlhng factors is less comphcated. The nature 

 of these factors is undetermined in these cases and their 

 consideration therefore would not at this time throw addi- 

 tional light on human problems. 



It shouki of course be held constantly in mind that 

 inheritance can be but one of the important influences 

 determining the incidence of any infectious disease. In the 

 case of tuberculosis as already outhned the factors we are 

 dealing with do not determine the absolute level of the 

 racial resistance. Such are at present intangible. What is 

 determined is the degree and kind of individual variation 



