PHENOMENA OF INHERITANCE 201 



certain diseases run in families and have the 

 appearance of being inherited, but in this case 

 as in many others it is extremely difficult in 

 the absence of experiments to distinguish be- 

 tween effects due to intrinsic causes and those 

 due to extrinsic ones. Where the specific 

 cause of a disease is some microorganism the 

 individual must have been infected at some 

 time or other, almost invariably after birth. 

 In few instances is the oosperm itself infected, 

 and even when it is, this is not, strictly speak- 

 ing, a case of inheritance, but rather one of 

 early infection. Pearson has found that there 

 is a marked correlation (represented by the 

 number .55 when complete correlation is 1.) 

 between tuberculous parents and tubercu- 

 lous children, but there is very little evidence 

 that the child is ever infected before birth. 

 What is inherited in this case is probably 

 slight resistance to the tubercle bacillus. There 

 is evidence that almost all adult persons have 

 been infected at one time or another by this 

 bacillus, but it has not developed far in all of 

 them because some have superior powers of 

 resistance. Such greater or smaller resistance, 



