130 NATURE AND LIFE. 



and extended analysis has thus resulted in an instructive 

 synthesis, which is one of the most signal acquisitions of the 

 experimental method. 



All animals have a temperature above that of the gas- 

 eous or fluid media in which they live ; that is to say, they 

 all possess the faculty of producing heat. Warm-blooded 

 animals maintain an almost constant temperature in all 

 latitudes and all climates. Thus, in polar regions, man, 

 mammals, and birds, mark only one or two degrees less than 

 they do at the tropics. The mean temperature of birds is 

 41 (cent.), and that of mammals 37. Those animals called 

 cold-blooded produce heat also, though in a less degree ; 

 but their temperature follows the variations of that of the 

 surrounding medium, keeping, however, a temperature a 

 few degrees higher than it. In reptiles, this excess varies 

 from 5 to half a degree ; in fish and insects, it is still 

 smaller ; and, in the wholly inferior species, it rarely reaches 

 half a degree. In fine, with animals that vary in tempera- 

 ture, the power of resistance to external causes of refrigera- 

 tion increases in proportion to the perfection of the organ- 

 ization. It is observed, too, that in these beings vital 

 activity and the force of respiration have a direct relation to 

 the thermometric state ; thus, in a medium of 7, lizards 

 consume eight times less oxygen than at 23. With ani- 

 mals of constant temperature, the reverse is the case ; the 

 colder it is, the more active is their respiration : a man, for 

 instance, who, in summer, consumes only a fraction over an 

 ounce of oxygen an hour, in winter consumes more than an 

 ounce and a half. Apart from the state of the surrounding 

 medium, many different circumstances exert a perceptible 

 influence on animal heat, and produce tolerably regular 

 variations in it. The seasons, the times of day, sleep, di- 

 gestion, mode of nourishment, age, etc., are thus constant 



