des Agens Physiques sur la Vie, 299 



to their infantile constitution. At first the sucking animal 

 shows little variation from the parent temperature ; then 

 this becomes more and more reduced, and about the fifteenth 

 day it is a degree or two below the mother. 



Birds, which are warmer than mammiferoe, were next 

 made the objects of experimental inquiry, and the young 

 recently hatched exhibited a lower temperature than the 

 grown birds. After removal from the shelter of their nests 

 into a mild atmosphere of 17°, in one hour they cooled down 

 from 36° to 19°, thus losing 17° in an hour. At an elevation 

 of 22° the same results were obtained, and they cooled down 

 to within one degree of the surrounding air. The plumage 

 of birds has little if any influence upon their temperature. 

 The production of heat lies within, and not on the surface 

 of the animal ; and if it be strongly developed, the removal 

 of natural coverings does not influence the heat produced ; 

 and if it be weak, their addition will not prevent cooling. 

 Birds recently escaped from the shell cooled to within two 

 degrees of the air, whereas the unplumed adult birds scarcely 

 lost one degree. 



The distinctive character of warm-blooded animals to pre- 

 serve an uniformity of heat has no reference to bulk. The 

 eagle maintains the same temperature as the wren or the 

 torn-tit, taking them at the same age, and placing them 

 under the same circumstances ; but if cooling measures be 

 adopted, the lesser body parts with its heat faster than the 

 larger, though ultimately they arrive at the same point. 

 The dimensions of animals are infinitely varied ; but the giant 

 reaches no higher standard than the dwarf, nor sinks to a 

 lower temperature. 



In estimating the temperature of young animals, it must 

 be taken into account that they are born at diflerent periods 

 of organic developement. Some come earlier into the world 

 than others, and some are more perfectly formed than others 

 at their birth, and more capable of helping themselves. 

 This variation produces a diflerent standard of heat after 

 birth, and especially creates a variety of temperature among 

 birds when tested at the same epochs of their existence. 

 The season in which animals are produced also modifies their 

 temperature. 



The influence of age in modifying temperature is common 

 both to mammiferae and birds. Young and healthy sucking 

 pigs cooled faster than their parent, their generatmg means 

 of heat being more feeble. Animals of warm blood possess 



