406 COMPARATIVE ZOOLOGY OF CHORDATES 



of the animal will vary by about io° C. Not only is the 

 constancy of the temperature less than that of higher mammals, 

 but the actual normal internal temperature is lower, being 

 about 30 C. In cold weather, Echidna hibernates. Its 

 protective covering of hair is poor, and, like a few other 

 mammals (such as the marmot), it becomes almost poikilo- 

 thermous. On the other hand, in hot weather when the 

 temperature rises above 35 C, Echidna dies of apoplexy 

 (unless it is activating, deep beneath the ground), for its only 

 method of countering a rise in the environmental temperature 

 is to reduce its own internal heat-production, and a point 

 is reached below which it cannot reduce its metabolism and 

 still live. 



The other Monotreme, Ornithorhynchus, has a slightly 

 higher normal temperature, 3 2° C, and it keeps it a little more 

 constant. Not only can it vary its heat-production, but it 

 can also vary its loss of heat by means of evaporation of water 

 from its sweat-glands. 



The higher mammals regulate their temperature almost 

 entirely by controlling the heat-loss. This they do by three 

 methods : by the evaporation of water from the sweat-glands, 

 by the dilatation of the blood-vessels in the skin, and by the 

 acceleration of respiration or ' ' panting. ' ' The heat-production 

 in these animals is not increased unless the external temperature 

 drops considerably. The Marsupials are intermediate between 

 the Monotremes and the higher mammals in the efficiency 

 of their temperature-regulations. 



In birds, heat is lost by evaporation of water through the 

 lungs and air-sacs. 



The advantages accruing from the possession of a high 

 and constant internal temperature are very great. Not only 

 does it allow of a higher rate of living, since chemical reactions 

 are accelerated at high temperatures, but it enables differentia- 

 tions and specialisations to arise which would be wrecked if 

 the speed of the metabolic processes (or in other words, the 

 internal temperature) were not constant. Further, it enables 

 the animals to inhabit climates in which poikilothermous forms 

 either cannot live, or have to spend considerable time hiber- 



