conditions for existence. The food of protozoa consists chiefly of
other organisms, particularly bacteria, and they are classed with the
animals. The protozoa are the most widely distributed and the most
universal of the parasites. The infectious diseases which they produce
in man, although among the most serious are less in number than those
produced by bacteria. So marked is the tendency to parasitism that
they are often parasitic for each other, smaller forms entering into
and living upon the larger. Variation does not seem to be so marked in
the protozoa as in the bacteria, though this is possibly due to our
greater ignorance of them as a class. We are not able, except in rare
instances, to grow them in pure culture, and study innumerable
generations under changes in the environment, as the bacteria have
been studied.

If we regard the living things on earth from the narrow point of view
as to whether they are necessary or useless or hostile to man, the
protozoa must be regarded as about the least useful members of the
biological society. It is very possible that such a conclusion is due
to ignorance; so closely are all living things united, so dependent is
one form of cell activity upon other forms that it is impossible to
foretell the result of the removal of a link. 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 o