THE SLEEPING SICKNESS 189 



host) may destroy them ; but, on the other hand, it may 

 lead to increased vigour and the most unexpected re- 

 action on their part in the production of virulent 

 chemical poisons. 



We are justified in believing that until man introduced 

 his artificially selected and transported breeds of cattle 

 and horses into Africa there was no nagana disease. The 

 Trypanosoma Brucei lived in the blood of the big game 

 in perfect harmony with its host. So, too, it is probable 

 that the sleeping-sickness parasite flourished innocently in 

 a state of adjustment due to tolerance on the part of the 

 aboriginal men and animals of West Africa. It was not 

 until the Arab slave raiders, European explorers, and india- 

 rubber thieves stirred up the quiet populations of Central 

 Africa, and mixed by their violence the susceptible with 

 the tolerant races, that the sleeping-sickness parasite be- 

 came a deadly scourge a " disharmony " to use the sug- 

 gestive term introduced by my friend Elias Metschnikow. 



The adjustment of primaeval populations to their con- 

 ditions has also been broken down by " disharmonies " of 

 another kind, due to man's restless invention, as explained 

 a few years ago in the interesting book of Mr. Archdall 

 Reid on the " Present Evolution of Man." Not only does 

 the human race within given areas become adjusted to a 

 variety of local parasites, but it acquires a tolerance of 

 dangerous drugs, such as alcohol and opium, extracted by 

 man's ingenuity from materials upon which he operates. 

 A race thus provided and thus immune imposes, by its 

 restless migrations, on unaccustomed races the deadly 

 poisons to the consumption of which it is itself habituated. 

 The unaccustomed races are deteriorated or even exter- 

 minated by the poisons thus introduced. 



Infectious disease, it was long ago pointed out, must 

 be studied from three main points of view : (i) the life 



