From the simplicity of structure it is not improbable that the bacteria are among the oldest forms of life, and all life has become adapted to their presence. They are of universal distribution; they play such an important part in the inter-relations of living things that it is probable life could not continue without them, at least not in the present way. They form important food for other unicellular organisms which are important links in the chain; they are the agents of decomposition, by which the complex substances of living things are reduced to elementary substances and made available for use; without them plant life would be impossible, for it is by their instrumentality that material in the soil is so changed as to be available as plant food; by their action many of the important foods of man, often those especially delectable, are produced; they are constantly with us on all the surfaces of the body; masses live on the intestinal surfaces and the excrement is largely composed of bacteria. It has been said that life would be impossible without 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.