RELATION TO ACTIVITY OF RESPIRATION. 249 



various classes of animals, as well as in individual ex- 

 amples of each, class, the quantity of heat generated in 

 the body is in direct proportion to the activity of the 

 respiratory process. The highest animal temperature, for 

 example, is found in birds, in whom the function of 

 respiration is most actively performed. In Mammalia, the 

 process of respiration is less active, and the average tem- 

 perature of the body less, than in birds. In reptiles, 

 both the respiration and the heat are at a much lower 

 standard ; while in animals below them, in which the 

 function of respiration is at the lowest point, a power of 

 producing heat is, in ordinary circumstances, hardly dis- 

 cernible. Among these lower animals, however, the 

 observations of Mr. Newport supply confirmatory evidence. 

 He shows that the larva, in which the respiratory organs 

 are smaller in comparison with the size of the body, has a 

 lower temperature than the perfect insect. Volant insects 

 have the highest temperature, and they have always the 

 largest respiratory organs and breathe the greatest quan- 

 tity of air ; while among terrestial insects, those also 

 produce the most heat which have the largest respiratory 

 organs and breathe the most air. During sleep, hyber- 

 nation, and other states of inaction, respiration is slower 

 or suspended, and the temperature is proportionately 

 diminished ; while, on the other hand, when the insect is 

 most active and respiring most voluminously, its amount 

 of temperature is at its maximum, and corresponds with 

 the quantity of respiration. Neither the rapidity of the 

 circulation, nor the size of the nervous system, according 

 to Mr. Newport, presents such a constant relation to the 

 evolution of heat. 



Similar evidence in favour of this theory of animal heat 

 is furnished by the fact that heat is sometimes evolved by 

 plants, in a quantity which appears to be in direct propor- 

 tion to the amount of oxygen they at the same time absorb 

 and convert into carbonic acid. For example, their evo- 



