of spores.  A fluid environment is essential to the life of the protozoa, but the resistant forms can endure long periods of dryness or other unfavorable environmental conditions.  The universal distribution of the protozoa is due to this; the spores or cysts can be carried long distances by the wind and develop into active forms when they reach an environment which is favorable.  Their distribution in water depends upon the amount of organic material this contains.  In pure drinking water there may be very few, but in stagnant water they are very numerous, living not on the organic material in solution in this, but on the bacteria which find in such fluid favorable 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.