508 THE RESPIRATORY EXCHANGE 



animals. The Darwinian theory leads to the conclusion that the 

 former class has arisen from cold-blooded ancestors, and experi- 

 ments upon the respiratory exchange and temperature of animals 

 have fully confirmed this conclusion ( 14 ). The temperature of newly 

 born pups, kittens, rabbits, and rats falls when they are deprived 

 of the warmth of their mother's body, and steadily declines until 

 it reaches a point a few degrees above the temperature of the air. 

 Death generally occurs when the internal temperature falls below 

 20. Newly born guinea-pigs, on the other hand, are able to 

 maintain their temperature, if the exposure to cold be not exces- 

 sive ; they differ from the other young animals in their great 

 development at birth, they are born well covered with fur, with 

 eyes open and such control over their limbs that they can run 

 about. The young rabbit, which may be taken as an example 

 of the other class, is born blind, helpless, and naked. Similar 

 differences are found among young birds ; the chick is able to 

 run about, see, peck up its food, and perform other complicated 

 movements within an hour or two of leaving its shell, and is well 

 covered with down ; the newly hatched pigeon is blind, helpless, 

 and naked. The chick can regulate its temperature, the pigeon 

 cannot. 



The young of warm-blooded animals can, therefore, be classi- 

 fied in two groups : the members of the first group respond to 

 changes of external temperature in a similar manner to that of 

 cold-blooded animals ; those of the second group resemble warm- 

 blooded animals, they increase their combustion when they are 

 exposed to cold. The evolution and development of this power 

 of regulation can be traced not only in the animal series, as shown 

 by the observations of Sutherland and C. J. Martin ( 15 ) upon mar- 

 supials and monotremes, but also in the same individual at different 

 stages of its existence. Hibernating mammals, moreover, have 

 apparently retained even in adult life some of the characteristics 

 of their cold-blooded ancestors ; during the summer they are active 

 and warm-blooded, during the winter they are torpid and cold- 

 blooded. 



Comparative determinations of the respiratory exchange in 

 these different animals demonstrate clearly these facts, and some 

 illustrative data may now be given. 



An adult mouse, by reason of its large cutaneous surface in 



