THE P HAS IS OF FORCE. 179 



functions are performed with a remarkable conformity to fixed 

 periods of time. Thus, in the incubated egg of the bird, which is 

 not left to casual supplies of warmth, but is constantly subjected 

 to the high temperature of the maternal body, the chick is matured 

 after a definite term of days ; and if the requisite heat were not 

 thus constantly supplied, not merely would the developmental 

 process be suspended, but the reduction of temperature would 

 annihilate the organizing power. For it is the peculiarity of warm- 

 blooded animals, that whilst this power is more energetic in its 

 action than in that of the lower tribes, it requires for its mainte- 

 nance a higher measure of heat; so that a reduction of the 

 temperature of the body to such a degree as would favour the 

 energetic activity of the lish or reptile, would be fatal to the bird 

 or mammal. 



Although there is still some obscurity respecting certain phe- 

 nomena of " animal heat," yet there is no question amongst 

 either chemists or physiologists in regard to the general fact, that 

 the main source of this heat is the oxygenation (by a kind of 

 combustive process) of the hydrocarbons contained in the food. 

 Now, we have seen that all these hydrocarbons, such as starch, 

 sugar, oil, etc., are either directly or indirectly derived from the 

 vegetable kingdom ; and not only a direct amount of oxygen, 

 hydrogen, and carbon have been consumed in their production, 

 but also a certain amount of solar light and heat, which they may 

 thus be said to embody. The combustive process is not so 

 carried on in the living body as to give forth light, save in a few 

 exceptional cases, but it reproduces in the form of heat all that 

 was embodied in the respiratory food, and thus the warm-blooded 

 animal may be said to be continually restoring to the universe 

 that force which the growing plant had appropriated to itself. 

 And, carrying the same principle a little further, we may say, that 

 in utilizing the stores of coal which have been prepared by the 

 luxuriant vegetation of past ages, man is not only restoring to the 

 atmosphere the carbonic acid and water of the carboniferous 

 epoch, but is actually reproducing and applying to his own use 

 the light and heat which its vegetation drew from the solar beams, 

 as if for the very purpose of fixing them until he should find the 

 means of turning them to account. Looking at this matter from 



