Certain elementary benefits from limited crowding of lower organisms may have been a large factor in the original development of the social instinct. In such diverse forms as protozoa, crustaceans, worms, marine and fresh water fish, and amphibians the formation of groups may be a protection against toxic substances dissolved in the liquid medium, the presence of noxious solids, extremes of temp. and other elements of bad environment. On the other hand, severe crowding is almost invariably harmful. The accumulation of metabolic waste products in a liquid medium decreases the longevity, growth and reproductive rate of animals living in it. Other noxious results of crowding are the decreased food supply and the mechanical disturbance of the individual. The effects of certain other environmental factors, particularly temp., on the speed of development, mortality, fecundity, and morphology of Drosophila are also reviewed.