bacteria, for the accumulation of the carcasses of all animals which have died would so encumber the earth as to prevent its use; but the folly of such speculation is shown by the fact that animals would not have been there without bacteria.  It has been shown, however, that the presence of bacteria in the intestine of the higher animals is not essential for life.  The coldest parts of the ocean are free from those forms which live in the intestines, and fish and birds inhabiting these regions have been found free from bacteria; it has also been found possible to remove small animals from their mother by Caesarian section and to rear them for a few weeks on sterilized food, showing that digestion and nutrition may go on without bacteria.

Certain species of bacteria are aerobic, that is, they need free oxygen for their growth; others are anaerobic and will not grow in the presence of oxygen.  Most of the bacteria which produce disease are facultative, that is, they grow either with or without oxygen; but certain of them, as the bacillus of tetanus, are anaerobic.  There is, of course, abundance of oxygen in the blood and tissues, but it is so combined as to be unavailable for the bacteria.  Bacteria may further be divided into those which are saprophytic or which find favorable conditions for life outside of the body, and the parasitic.  Many are exclusively parasitic or saprophytic, and many are facultative, both conditions of living being possible.  It has been found possible by varying in many ways the character of the culture medium and temperature to grow under artificial conditions outside of the body most, if not all, of the bacteria which cause disease.  Thus, such bacteria as tubercle bacilli and the influenza bacillus can be cultivated, but they certainly would not find natural conditions which would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances in the fluid in which they are growing.  Growth may be inhibited by the smallest trace of some of the metallic salts, as corrosive sublimate, although the bacteria themselves are not destroyed.  If small pieces of gold foil be placed on the surface of prepared jelly on which bacteria have been planted, no growth will take place in the vicinity of the gold foil.

Variations can easily be produced in bacteria, but they do not tend to become established.  In certain of the bacterial species there are strains which represent slight variations from the type but which are not sufficient to constitute new species.  If the environment in which bacteria are living be unusual and to a greater or less degree unfavorable, those individuals in the mass with the least power of adaptibility will perish, those more resistant and with greater adaptability will survive and propagate; and the peculiarity being transmitted a new strain will arise characterized by this adaptability.  Bacteria with slight adaptability to the environment of the tissues and fluids of the