The inadequacy of mere morphology as a basis for describing bacteria led to the search for diagnostic characters, based on the biochemical changes that they produced in their culture media, and the appearance of their growth in the mass on various media. These characters unfortunately have also proved to be very much influenced by the exact composition of the medium and other conditions of culture. Recently an attempt has been made by the American Society of Bacteriologists to standardise the diagnostic characters used in describing bacteria, and also the media and cultural conditions under which they are grown for the purpose of description. The need for such precautions, however, was not sufficiently realised by the early workers, many of whose descriptions cannot now be referred to any definite organism. The large number of organisms found in the soil, and the difficulty and labour of adequately describing them, is such that even now we have no comprehensive description of the common soil bacteria that appear on gelatine platings. A careful study based on modern methods of characterisation has been made of certain selected groups of bacteria, and it is hoped that the laborious systematic work of describing the common forms will gradually be completed. Several attempts have been made to classify the bacteria that appear commonly on gelatine platings. This work was commenced by Hiltner and Stormer in Germany, and continued by Chester, Harding, and Conn in America. Conn[10],[14] found that the common organisms fell into the following main groups:-- (1) Large spore-forming bacteria, related to _Bacillus subtilis_, which form about 5-10 per cent. of the numbers. He adduced evidence[12],[13] that these organisms exist in the soil mainly as spores, so that they may not form an important part of the active soil population. (2) Short non-sporing organisms, related to _Pseudomonas fluorescens_, that are rapid gelatine liquefiers. These form another 10 per cent. of the numbers. (3) Short rod forms that liquefy gelatine slowly or not at all, and develop colonies very slowly. These form 40-75 per cent. of the numbers, and may therefore be of considerable importance in the soil. (4) A few micrococci also occur. These groups comprise the larger portion of the bacterial flora of the soil, but, in addition to these organisms, that develop on the media commonly used for plating, there are special and important groups that appear only on special media, either owing to their being unable to grow on ordinary media or because they get swamped by other forms. Examples of such groups are the ammonia and nitrite oxidising bacteria, the nitrogen fixing groups, the cellulose decomposing organisms, and the sulphur bacteria.