40 HOMOIOTHERMISM 



to life at extreme temperatures, but in general the state- 

 ment at the beginning of this paragraph is true. 



At lower temperatures a poikilothermic animal often 

 has a body temperature which is somewhat above that of 

 the environment. The tunny may thus show a tempera- 

 ture more than 3 deg. C. above that of the surrounding 

 water (Murray, 1914). In turtles, Baldwin (1925) found 

 a non-critical range between 10 to 27 deg. C, where body 

 temperatures were at times from 1.5 to 3 deg. C. above 

 the surroundings, but at lower temperatures, and espe- 

 cially at about 4.5 deg. C, the body temperature showed 

 a marked tendency to remain higher than the environ- 

 ment. He believes there is thus '*a slight tendency to 

 compensate for critical changes in the environment." 

 McClendon (1918) and others who have studied the rela- 

 tions of temperature inside and outside various inverte- 

 brates and aquatic vertebrates find no such tendency. 

 This is quite a contrast to conditions in the body of a 

 mammal. A man, for example, has a high degree of 

 ability to compensate for environmental changes in tem- 

 perature. He maintains a constant temperature and 

 gives off no more heat at 18 deg. C. in a room under a 

 blanket than when in water having a temperature of 37 

 deg. C. (Benedict, 1914). In passing from fishes through 

 the various classes of vertebrates to mammals and birds 

 there is increasing control of rate metabolism and body 

 temperature by nervous mechanisms. For example, the 

 rate of respiration in a turtle is controlled largely by 

 the temperature of the animal's neck. The temperature 

 of the blood going to the head affects the respiratory 

 centers in the brain. The temperature of the center cells 

 is thus a more potent influence than the general activity 

 or temperature of the body as a whole (Lumsden, 1924). 

 In mammals the presence of carbon dioxide in the blood 



