MAINTENANCE OF TEMPERATURE. 245 



ture of the others, which we call cold-blooded, is " variable 

 with every atmosphere." 



The power of maintaining a uniform temperature, which 

 Mammalia and birds possess, is combined with the want 

 of power to endure such changes of temperature as are 

 harmless to the other classes ; and when their power of 

 resisting change of temperature ceases, they suffer serious 

 disturbances or die. M. Magendie has shown that birds 

 and rabbits die when, being exposed to great external 

 heat, their temperature is raised as much as 9 above the 

 natural standard : but that they bear a reduction of the 

 temperature of the interior of the body to a much greater 

 amount before very dangerous or fatal consequences ensue. 



In all the ordinary circumstances of life, the maintenance 

 of uniform temperature is effected by the production of 

 heat sufficient to compensate for that which is constantly 

 lost in radiation into the medium in which we live, or in 

 combination with the fluids evaporating from the exposed 

 surfaces of the body. 



The losses thus sustained are extremely various in 

 different circumstances ; and the degrees of power which 

 animals possess of adapting themselves to such differences 

 are equally various. Some live best in cold regions, where 

 they produce abundant heat for radiation, and cannot 

 endure the heat of warm climates, where the heat that 

 they habitually produce would, probably, be excessive, 

 and by its continual, though perhaps small excess, would 

 generate disease ; others, naturally inhabiting warm cli- 

 mates, die if removed to cold ones, as if because their 

 power of producing heat were not quite sufficient to com- 

 pensate for the constantly larger abstraction of it by 

 radiation. Man, with the aid of intellect for the provision 

 of artificial clothing, and with command over food, is, in 

 these respects, superior to all other creatures ; possessing 

 the greatest power of adaptation to external temperature, 

 and being capable of enduring extreme degrees of heat, 



