The protozoa do not seem to be as necessary for the life of man as are the bacteria; they produce many of the diseases of man, many of the diseases of animals on which man depends for food; they cause great destruction in plant life, and in the soil they feed upon the useful bacteria.

It is well to remember, however, that fifty years ago several of the organs of the body whose activity we now recognize as furnishing substances necessary for life were regarded as useless members and, since they became the seat of tumors, as dangerous members of the body.

The only organ which now seems to come into such a class is the vermiform appendix, and its lowly position among organs is due merely to an unhappy accident of development.
The class of organisms known as the filterable viruses or the ultra-microscopic or the invisible organisms have a special interest in many ways.

The limitation in the power of the microscope for the study of minute objects is due not to a defect in the instrument but to the length of the wave of light.

It is impossible to see clearly under the microscope using white light, objects which are smaller in diameter than the length of the wave which gives a limit of 0.5 mu.