44: THE DATA OF BIOLOGY. 



and genesis of heat once begin in them. Whereas among 

 the second, there universally exists the ability, more or less 

 decided, thus to evolve heat ; and the evolution of heat, in 

 some cases very slight and in no cases very great, continues 

 as long as they remain animate bodies. 



The relation between active change of matter and re-active 

 genesis of atomic vibration, is clearly shown by the contrasts 

 between different organisms, and between different states and 

 parts of the same organism. In plants, the genesis of heat is 

 extremely small, in correspondence with their extremely 

 small production of carbonic aid : those portions only, as 

 flowers and germinating seeds, in which considerable oxidation 

 is going on, having a decidedly raised temperature. Among 

 animals, we see that the hot-blooded are those which expend 

 much force and respire actively. We see that though such 

 creatures as insects are scarcely at all warmer than the surround- 

 ing air when they are still, they rise several degrees above it 

 when they exert themselves ; and that in creatures like our- 

 selves, which habitually maintain a heat much greater than 

 that of their medium, exercise is accompanied by an ad- 

 ditional production of heat, often to an inconvenient extent. 



This molecular agitation accompanying the molecular 

 re-arrangements that are caused by oxygen taken into the 

 animal organism, must result both from the union of oxygen 

 with those nitrogenous matters of which the tissues are 

 composed, and from its union with those non-nitrogenous 

 matters which are diffused through the tissues. Just as much 

 heat as would be caused by the oxidation of such matters 

 out of the body, must be caused by their oxidation in the 

 body. In the one case as in the other, the heat must be re- 

 garded as a concomitant. Whether the distinction 

 made by Liebig between nitrogenous substances as tissue- 

 food, and non-nitrogenous substances as heat-food, be true or 

 not in a narrower sense, it cannot be accepted in the sense 

 that tissue-food is not also heat-food. Indeed he does not 

 himself assert it in this sense. The ability of carnivorous 



