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 indiarubber 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 became a deadly scourge--a “disharmony” to use the suggestive term introduced by my friend Elias Metschnikow. The adjustment of primaeval populations to their conditions 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 exterminated by the poisons thus introduced.