The importance of the organic matter present in the soil is well illustrated in the table on p. 152. The great increase in the number of insects and other animals is partly due to their direct introduction along with the manure, and partly to their entry into the soil in response to chemotropic stimuli exerted by fermentation. Organic matter influences the fauna in other ways also; it increases the moisture content of the soil, and it provides many species with an abundance of food material. Also, the amount of carbon dioxide present in the soil is partly dependent upon decaying organic matter. Hamilton conducted experiments on the behaviour of certain soil insects in relation to varying amounts of carbon dioxide. Although his work is of too limited a nature to be accepted without reserve, it lends support to the conclusions of Adams who says: “The animals which thrive in the soil are likely to be those which tolerate a large amount of carbon dioxide, and are able to use a relatively small amount of oxygen, at least for considerable intervals, as when the soil is wet during prolonged rains. The optimum soil habitat is therefore determined, to a very important degree, by the proper ratio or balance between the amount of available oxygen and the amount of carbon dioxide which can be endured without injury.” Little is known concerning the occurrence of ammonia in the soil atmosphere, but its presence in minute quantities is probably an important chemotropic factor in relation to saprophagous organisms which are the largest constituent of the fauna. A great increase in Dipterous larvæ occurs on the addition of farmyard manure, and this is noteworthy in the light of Richardson’s experiments (1916), which indicate that ammonia exercises a marked attraction for _Diptera_, which spend some part of their existence in animal excrement in some form or another. The nature of the vegetation supported by the soil is of paramount importance in relation to phytophagous organisms, and examples need scarcely be instanced of certain species of soil insects being dependent upon the presence of their specific food plants. THE RELATION OF SOIL INVERTEBRATES TO AGRICULTURE. The relation of these organisms to agriculture may be considered from three points of view: (_a_) their influence upon the soil itself; (_b_) their relation to the nitrogen cycle; and (_c_), their direct influence upon economic plants.