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 this is by budding in which projections or buds form on the body and after separation become new organisms.  In other cases spores form within the cell which become free and develop further into complete organisms.  These simple modes of multiplication often alternate in the same organism with sexual differentiation and conjugation.  There is never a permanent sexual differentiation, but the sexual forms develop from a simple and non-sexual organism.  Usually the sexual forms develop only in a special environment; thus the protozoon which in man is the cause of malaria, multiplies in the human blood by simple division, but in the body of the mosquito multiplication by sexual differentiation takes place.  Under no conditions is multiplication so rapid as with the bacteria, and in general the simpler the form of organism the more rapid is the multiplication.  It is common to all of the protozoa to develop forms which have great powers of resistance, this being due in some cases to encystment, in which condition a resistant membrane is formed on the outside, in others to the production