In an environment of temps. far higher than those tolerable within the body, the most heat enduring desert reptiles known possess optimum temps. only 3[degree]C-4[degree]C below the point of heat induced discomfort. These animals tolerate temps. as much as 9oC below their optimum and will survive temps. as much as 20oC lower. Ancestral reptiles may have responded in a similar manner and likewise have been limited by 45[degree]C, the known metazoan maxi- mum protoplasmic tolerance. According to Liebig''s law this would place the reptiles in graver danger from a rise rather than a fall in temps. Many characteristics of archo-saurs can be interpreted as affording protection from excessive heat and likewise as interposing handicaps at low temps. Under super hot conditions increments in volume provide a favorable trend in surface-mass ratios. The development of non-vascular or nearly non-vascular structures such as bone, scales, feathers and hair favours the poikilotherm encountering excessive radiant heat, but interposes thermal handicaps at low temps. Noctumalism and resort to subterranean retreats are likewise practical methods of evading excessive heat. The inadequacy of the terms poikilotherm and homoiotherm suggests the substitution of ectotherm with its subdivisions heliotherm and thigmotherm in place of poikilotherm, and endotherm in place of homoiotherm.