92 HOMOIOTHERMISM 



when the environment varies from 5 to 35 deg. C. Dur- 

 ing cold weather Echidna abandons all attempts at 

 homoiothermism and hibernates for four months with 

 practically no difference between its body temperature 

 and that of the surroundings. At high temperatures 

 Echidna does not increase the number or depth of its 

 respirations. It possesses no sweat glands and exhibits 

 no evidence of ability to vary loss of heat by vaso-motor 

 adjustment in response to changes in external tem- 

 peratures. 



Ornithorhynchus, which is also a primitive homoio- 

 therm, is distinctly above Echidna, for, although its body 

 temperature is low, it is fairly constant. This animal 

 possesses numerous sweat glands upon the soft snout 

 and frill but has none elsewhere on the body. Martin 

 found that carbon dioxide production varied with the 

 external temperature and this indicates that the animal 

 can modify heat-loss as well as heat-production. Orni- 

 thorhynchus does not increase the rate of its respirations 

 at high temperatures. Marsupials, however, show evi- 

 dence of utilizing variation in heat-loss to a greater ex- 

 tent than it does, but less than higher mammals. Their 

 respiratory rates are sometimes slightly increased at 

 high temperatures. 



Variation in production of heat is the ancestral 

 method of regulating body temperature. Through de- 

 veloping a mechanism by means of which an organism 

 may vary the production of heat in accordance with heat 

 lost, animals have overcome the one great disadvantage 

 of cold-bloodedness — activity is no longer dependent 

 upon external temperature. At this stage a homoiotherm 

 has also increased its ability to remain active at low tem- 

 peratures. Later, by developing a mechanism control- 

 ling loss of heat, it increases its range in the direction 



