n be
cultivated, but they certainly would not find natural conditions which
would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances
in the fluid in which they are growing. Growth may be inhibited by the
smallest trace of some of the metallic salts, as corrosive sublimate,
although the bacteria themselves are not destroyed. If small pieces of
gold foil be placed on the surface of prepared jelly on which bacteria
have been planted, no growth will take place in the vicinity of the
gold foil.

Variations can easily be produced in bacteria, but they do not tend to
become established. In certain of the bacterial species there are
strains which represent slight variations from the type but which are
not sufficient to constitute new species. 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. Pasteur found that by gradually increasing the temperature
they could be grown at one hundred and ten degrees. When grown at this
temperature they were no longer so virulent and produced in animals a
mild non-fatal form of anthrax which protected the animal when
inoculated with the virulent strain. The well known variations in the
character of disease, shown in differences in severity and ease of
transmission, seen in different years and in different epidemics, may
be due to many conditions, but probably variation in the infecting
organisms is the most important.

The protozoa, like the bacteria, are unicellular organisms and contain
a nucleus as do all cells. They vary in siz