It must be borne in mind that these estimates only apply to average conditions; the outbreak of a plant pest in any one year must naturally materially alter the proportions given. The phytophagous organisms are represented by a certain number of the _Insecta_ together with the pulmonate _Mollusca_. Carnivorous forms which are mainly beneficial from the agricultural standpoint, include _Insecta_, together with the _Chilopoda_, many _Acarina_ and the _Areinida_. Saprophagous forms constitute the dominant element of the soil fauna. More than 30 per cent. of the _Insecta_ exhibit this habit, which is also the dominant one in the _Oligochæta_, _Symphyla_, and in many of the soil _Nematodes_. Heterophagous species include all those of somewhat plastic habits; for the most part they are saprophagous, but, on the other hand, a considerable proportion of the species attack growing plants or exhibit both habits. Under this category are included a certain number of the _Insecta_, the _Diplopoda_, _Isopoda_, and some _Acarina_. THE INFLUENCE OF ENVIRONMENTAL FACTORS UPON THE INVERTEBRATES OF THE SOIL. Since animals are endowed with powers of independent locomotion: they are not necessarily tied to their environment to the same extent that plants are. The investigation of the influence of environmental factors sooner or later involves a study of the tropisms of the animals concerned. Until these are adequately understood it is scarcely possible to arrive at any exact conclusions relative to their behaviour in the soil. Insects, for example, respond to the stimuli of various, and often apparently insignificant forces, acting upon their sensory organs. Such responses are known as chemotropism, phototropism, hydrotropism, thermotropism, and so forth according to the nature of the stimuli. Tropisms are automatic and, so far as they distinguish sensations, are independent of any choice, and consequently of psychic phenomena. Animal automatism, however, does not present the rigidity of mechanical automatism. Differential sensibility, vital rhythms, or periodicity, etc., are other important aspects of animal behaviour.