EFFECT OF EXTERNAL TEMPERATURE 509 



comparison with, its mass, responds very rapidly to a change of 

 external temperature. 



The relationship ( 16 ) between muscular activity and the pro- 

 duction of carbon dioxide and heat is well marked ; in cold 

 surroundings the mouse is very active, whereas with a warm 

 external temperature it is quiet and often goes to sleep. 



Young mice one day old show a decrease in the output of 

 carbon dioxide when the external temperature is changed from 

 30 to 20 ; within half-an-hour the decrease may be 50 per cent, 

 of the original value and the temperature of their bodies falls, 

 in some cases as much as 9. At this early age the mice respond 

 for higher temperatures in a manner similar to that seen in adult 

 animals ; a rise in external temperature from 30 to 40 reduces 

 the discharge of carbon dioxide to about one-half of its previous 

 value and raises their temperature. A chick during its develop- 

 ment passes first through, a cold-blooded stage, then through an 

 apparently neutral stage, and when it is hatched responds as a 

 warm-blooded animal to changes of external temperature. The 

 marked contrast exhibited by the young pigeon during the first 

 few days after it is hatched is shown by the following figures (see 

 p. 510). 



As in the case of the young mice, a rise in external tempera- 

 ture to 40 determines a decrease in the output of carbon dioxide 

 and a rise in the temperature of the young pigeon. 



