Temperature is the most important single factor governing the occurrence and behavior of life. Temperature changes affect protoplasm directly and change the physical and biological environment as well. Protoplasm is destroyed at 100°C. and when ice crystals form within it. Some plants live in hot springs where the temperature is at least 85°C, and some deep-sea animals live at perpetual subzero temperatures, but these extremes encompass a relatively narrow range (Fig. 1). There is a great mass of information available concerning temperature as an ecological factor, but its actual operation is sometimes difficult to evaluate. This is because many of the “temperature” data are simply positions on a column of mercury, without consideration of gradients or duration. The rate of change, or temperature gradient, may be as important to the ecology of an organism as its total summation, its average, or its actual range within a given environment during a period of time. TEMPERATURE AND THE PHYSIOLOGY OF ORGANISMS Resistance to chemical reactions is lowered by increase in temperature, hence a rise in temperature speeds up reactions. Biological systems are heterogeneous and involve physical as well as chemical processes. The velocity of reactions in protoplasm is determined by the factor having the slowest rate, which may often be diffusion. For this reason many life processes are not accelerated by a temperature rise as much as chemical reactions are, but the correspondence between temperature coefficients of chemical and biological reactions is often close. At the temperature range at which life processes...