298 HEREDITY AND DISEASE 



Of course, this is a very difficult question, in regard to which 

 no one would wish to dogmatise. But one must not too readily 

 assume that the correct interpretation is the hereditary trans- 

 mission of acquired immunity, (i) It is possible that the 

 micro-organisms concerned are evolving in the direction of 

 attenuated virulence. (2) Much may be due to improved 

 treatment. Thus " measles " may not be really milder, but 

 simply better treated. (3) The result may be in part due to an 

 ehmination of the most susceptible, which leaves the race as a 

 whole more resistent. (4) There may be a quite independent 

 widespread variational change in the direction of more resisting 

 power to these two diseases — a germinal variational change 

 quite apart from the ordeal of infection. (5) The power of 

 resistance may be improved by diet ; thus measles is often most 

 acute in out-of-work winters, and least acute when the nutritive 

 conditions are good. (6) The severe so-called " types " of cer- 

 tain diseases were probably " mixed infections." Nowadays 

 there is perhaps a greater number of " pure infections." (7) 

 Some of the more virulent germs are probably being stamped 

 out. There is no proof that the germ of the now somewhat rare 

 scarlatina maligna is the same race as that of the common 

 scarlatina simplex. (8) It is possible that the mothers may 

 through the placenta confer their own acquired immunity on 

 their offspring. 



Natural immunity is a well-known inborn peculiarity, some- 

 times racial, sometimes personal, and manifested in various 

 degrees. Negroes are relatively immune to yellow fever and 

 ague ; Algerian sheep are relatively immune to anthrax ; certain 

 individuals appear to enjoy peculiar immunity in the midst of 

 epidemics. 



It is generally believed that racial immunity has been gradually 

 wrought out in the course of natural selection. Germinal varia- 

 tions in the direction of immunity enable their possessors to 

 survive ; the survivors transmit their refractory constitution ; the 



