THE PRKSENT EVOLUTION OF MAN— PHYSICAL 215 



other diseases, but experience proves that this is not to 

 any extent the case. An attack of measles, for instance, 

 does not at all protect against scarlatina. But when 

 we learn that acquired immunity against any disease is 

 due to a power acquired by the phagocytes of with- 

 standing the toxins of that disease, and when we 

 remember, in this connection, that the toxins of any 

 disease must be different from the toxins of every 

 other disease, since their effects on the organism are so 

 different, we are able to understand why acquired 

 immunity against any one disease holds good against 

 no other. 



