In the initial stages of soil formation during the disintegration and decomposition of rocks, the first type of soil to be formed is suitable for the growth of only certain plants; it is of a purely mineral nature, containing raw food materials derived mainly from the rocks and not from organic matter, unless from such organic residues as were incorporated in the rocks during their formation in ancient times. Such soil cannot sustain the higher types of green plants, nor is it populated by soil organisms; it furnishes suitable pabulum, however, for the nourishment and growth of the more lowly types of vegetation, which are able to convert to their benefit the limited supply of food materials available. The complex organic compounds that such primitive plants elaborate from these food materials of purely mineral origin, and incorporate in their tissues, are, after death, returned to the soil, which becomes correspondingly enriched, and a favourable environment for the establishment of organisms; the latter reduce these plant residues to humus, and during this process of decomposition produce food materials of an organic origin suitable for the nutrition of the sequential plant covering. So the process proceeds until a soil is formed of sufficient extent and quality for the support of a more extensive and increasingly complex vegetation; thus, in the cycle of life and decay, stores of organic compounds are elaborated by plants and returned to the soil, which they enrich, and where they are decomposed by organisms, and so maintain the supplies of food materials suitable for the maintenance of vegetation. These phenomena of plant establishment and succession, correlated with soil formation, were clearly demonstrated by the re-establishment of vegetation after the soil and plant life had been destroyed by the historic eruption in 1883 of Krakatoa, a volcanic island in the Straits of Sunda, between Java and Sumatra. The first plants to be established on the volcanic deposits were species of terrestrial algæ, which gradually spread and built up soil suitable for the development of soil organisms and for the growth of seeds brought to the island by birds and ocean currents. So rapid were the changes brought about by these influences, that within a period of twenty years after the eruption the barren ground was reclothed by a dense and varied plant covering. Organisms that form part of the organic complex of the soil range from the more conspicuous species, such as slugs and snails, insects, spiders, wood lice, millepedes, earthworms and eelworms, to such microscopic forms as protozoa, fungi, algæ and bacteria, the last three being members of the plant kingdom. These organisms may be grouped as follows:-- (1) Temporary inhabitants that enter the soil for shelter, or to feed as scavengers on decaying organic matter, or both--_e.g._, slugs, snails, wood lice, certain insects and some eelworms.