5G 



necessarily vary between very remote limits, viz. — those of 

 the climate at the different seasons of the year. 



The amount of deviation from the temperature of the 

 external air, in moderate temperatures, is different in the 

 different individuals of this division, and is the greatest in 

 those who possess most rapidity and power of action, and 

 varies in the same animal at different seasons, in different 

 climates, and at different ages. Contrary, however, to what 

 we find in warm-blooded animals, the heat-producing power, 

 measured by the excess of its temperature above the 

 external medium, is promoted by the warmth of the season 

 or climate, whereas in the latter it is promoted by the cold. 

 Intermediate between the two great classes there exists in 

 the mammalia a smaller class — the hybernating animals, 

 which may be said to belong at one season to the warm- 

 blooded and at another to the cold-blooded animal. They 

 at no time however can, when in repose, bear the influence 

 of a considerable depression of the external temperature 

 without a very great depression of their own ; and in 

 their most active stage, they resemble the young of those 

 warm-blooded animals, which are born in an imper- 

 fectly developed condition, viz. — the animals born with 

 their eyes closed, and birds born without feathers. They 

 resemble the cold-blooded animals in this, that contrary to 

 what occurs in other warm-blooded animals, heat promotes 

 the functional power. In the preceding part of this paper 

 I have attempted to point out, from the best authorities, the 

 amount of heat formed in different animals, and the physical 

 and physiological circumstances under whose influence these 

 vary. In the continuation which I hope to be able to lay 

 before the Society, I will examine the various theories 

 which, from the time of Black, Lavoisier, and Crawford, up 

 to the present day, have been offered to explain the chemical 

 changes taking place in the body, the sources of animal 

 heat, &c. 



