The bacteria are divided into species, the classification being based on their forms, on the mode of growth, the various substances which they produce and their capacity for producing disease.  The differentiation of species in bacteria is based chiefly upon their properties, there being too little difference in form and size to distinguish species.  The introduction of methods of culture was followed by an immediate advance of our knowledge concerning them.  This method consists in the use of fluid and solid substances which contain the necessary salts and other ingredients for their food, and in or on which they are planted.  The use of a solid or gelatinous medium for growth has greatly facilitated the separation of single species from a mixture of bacteria; a culture fluid containing sufficient gelatine to render it solid when cooled is sown with the bacteria to be tested by placing in it while warm and fluid, a small portion of material containing the bacteria, and after being thoroughly mixed the fluid is poured on a glass plate and allowed to cool.  The bacteria are in this way separated, and each by its growth forms a single colony which can be further tested.  It is self-evident that all culture material must be sterilized by heat before using, and in the manipulations care must be exercised to avoid contamination from the air.  The refraction index of the bacterial cell is so slight that the microscopic study is facilitated or made possible by staining them with various aniline dyes.  Owing to differences in the cell material the different species of bacteria show differences in the facility with which they take the color and the tenacity with which they retain it, and this also forms a means of species differentiation.  The interrelation of science is well shown in this, for it was the discovery of the aniline dyes in the latter half of the nineteenth century which made the fruitful study of bacteria possible.

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