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 size from forms seen with
difficulty under the highest power of the microscope to forms readily
seen with the unaided eye. Their structure in general is more complex
than is the structure of bacteria, and many show extreme
differentiation of parts of the single cells, as a firm exterior
surface or cuticle, an internal skeleton, organs of locomotion, mouth
and digestive organs and organs of excretion. They are more widely
distributed than are the bacteria, and found from pole to pole in all
oceans and in all fresh water. There are many modes of multiplication,
and these are often extremely complicated. The most general mode and
one which is common to all is by simple division; a modification of