All environmental factors at their tolerance extremes become critical to the organism, affecting its survival. I venture to suggest that among these factors, temperature, as the master factor, is the most critical. As a lethal factor it destroys the integrity of the organism and kills. Within the tolerance zone, temperature acts as a controlling factor and regulates metabolism and all types of activity, and functions including behavior, for example preference. Temperature sets its tolerance range and optimum. Its effects are modified through the process of acclimation as well as by interactions with the other environmental factors.