INHERITANCE IN MANKIND 49 



When the condition is brought into the pedigree from 

 several sides, it usually becomes more persistent, and we 

 get such terrible results as those shown below. 



1 /6 T 9 



I I I 



' m* -A. A 



ffl /A ^A A ^ dl 



Here every member of the second and third 

 generations is a deaf-mute. 



Even with diseases which are infectious, a predis- 

 position to the disease is often or always an hereditary 

 character. Let us take as an example tuberculosis, 

 which still kills about one in ten of our population. 

 Of late years, stress has been laid on the infectious 

 nature of this scourge, and there has been a tendency 

 to overlook the effect of heredity in transmitting a 

 nature prone to its attacks. Yet a glance at such a 

 pedigree as that given below must recall us to the 

 study of ancestral influences. 



The individuals i, 2, and 3 of generation V., and 

 i of generation VI., have not yet arrived at the age 

 when tuberculosis usually begins. Excluding these 

 cases; and the doubtful cases I. i and V. 7, we have 

 17 individuals descended from our first pair. Of 

 these descendants 15 are certainly tuberculous, and 

 only 2 are classed as free from the disease. 



Such pedigrees as we have considered are sufficient 

 to suggest the preponderating effect of heredity in 



