(_a_) The behaviour of earthworms as a factor inducing soil fertility is discussed by Darwin in his well-known work on the subject, and their action may be briefly summarised as follows. In feeding habits they are very largely saprophagous, and consume decaying vegetable matter including humus, which they swallow, together with large quantities of soil. Earthworms come to the surface to discharge their fæces (“worm casts”), and in this process they are continually bringing up some of the deeper soil to the air. Darwin estimated that earthworms annually brought to the surface of the soil in their “casts” sufficient earth to form a layer ·2 inch in depth, or 10 tons per acre. Their action, along with the atmosphere, are the chief agencies which produce the uniformity and looseness of texture of the surface soil. By means of their burrows earthworms facilitate the penetration of air and water into the soil, while their habit of dragging leaves and other vegetable material into these burrows increases the organic matter present below the surface. These facts are generally agreed upon, but it is a disputed point whether earthworms, by devouring organic matter, aid the conversion of the latter into plant food more rapidly than takes place solely through the activities of micro-organisms. Soil insects and other arthropods, by their burrowing activities, are also instrumental in loosening the soil texture and thereby facilitating soil aeration and the percolation of water. The action of termites in warmer countries is discussed by Drummond in his “Tropical Africa,” who compares the rôle of subterranean termites to that of earthworms. The great abundance of ants renders them also significant in this same respect, and very few species are direct enemies of the agriculturist. [Illustration: FIG. 23.--Diagram showing the Relation of the Soil Invertebrata (other than Protozoa) to the Nitrogen Cycle.]