PHYSIOLOGICAL adaptation of the rates of life processes in response to changes in environmental temperature is now known to be commonplace in poikilothermic organisms1. Animals, plants and micro-organisms experimentally subjected to a change in temperature within normal viable limits initially show modified rates of metabolic functions which, in the absence of “overshoot”, accord with predictions based on chemical kinetics. In many cases, however, these rates either decrease (rise in temperature: “T+” adaptation) after the first phase or increase (fall in temperature: “T−” adaptation) slowly over a period of days or weeks, eventually attaining a new constant value which tends towards the rate characteristic for the original temperature. The nearer the correspondence the more highly adaptable is the organism in that particular respect2. Such slow modifications are generally considered to be true physiological adaptation (that is, “capacity adaptation”) in contrast to the rapid changes (overshoots) which are completed in a matter of minutes or hours after the change in temperature.