The environmental factors, affecting more especially the insect population of the soil, have been discussed by Cameron (1917) and Hamilton (1917), and certain broader aspects of animal ecology by Adams (1915) and Shelford (1912). These factors are so numerous and so inter-connected, that it is only possible to refer to them briefly in the space available. As might be expected, soils that are of a light and open texture are the ones most frequented by soil insects, nutritional and other factors being equal. Furthermore, it has already been shown that in arable land insects and other animals penetrate to a greater depth than in pastures. This fact is primarily due to the greater looseness of the soil occasioned by agricultural operations, which ensure at the same time better drainage aeration, and greater facilities for penetration. Hamilton found that soil insect larvæ are very sensitive to evaporation, and especially so if the temperature is 20° C. or over. In their natural habitat the relative humidity of the air, in moist or wet soil, is not far below saturation, and the temperature of the soil rarely goes above 20°-23°C., and then only in exposed, dry, hard soil in which these larvæ do not occur. The significance of the rate of evaporation as an environmental factor was first emphasised by Shelford. According to him the best and more accurate index of the varying physical conditions affecting land animals, wholly or in part exposed to the atmosphere, is the evaporating power of air. By means of a porous cup-atmometer, as devised by Livingston, Shelford has carried out an important series of experiments on the reactions of various animals to atmospheres of different evaporation capacities, and reference should be made to his text-book.