If the environment in which bacteria are living be unusual and to a greater or less degree unfavorable, those individuals in the mass with the least power of adaptibility will perish, those more resistant and with greater adaptability will survive and propagate; and the peculiarity being transmitted a new strain will arise characterized by this adaptability.

Bacteria with slight adaptability to the environment of the tissues and fluids of the animal body can, by repeated inoculations, become so adapted to the new environment as to be in a high degree pathogenic.

In such a process the organisms with the least power of adaptation are destroyed and new generations are formed from those of greater power of adaptation.

When bacteria are caused to grow in a new environment they may acquire new characteristics.

The anthrax bacilli find the optimum conditions for growth at the temperature of the animal body, but they will grow at temperatures both above and below this.