221 



the heat of fishes, and the decline of animal heat 

 which the amphibia (176.), when exposed to great 

 cold, experience, all demonstrate, that the surround- 

 ing medium, whether it be air or water, is constant- 

 ly drawing off their heat, which renders necessary 

 as constant a reproduction of it. 



179. By what process, then, or from what source, 

 is this superiority of temperature derived ? It does 

 not, says Mr Hunter, depend on the motion of the 

 blood, as some have supposed, because it likewise 

 belongs to animals which have no circulation : neither 

 can it be said to depend on the nervous system, for 

 it is found in animals which have neither brain nor 

 nerves. * It is probable, he adds, that it arises from 

 some principle, so connected with life, that it can 

 and does act independently of circulation, sensation r 

 and volition ; and is that power which preserves and 

 regulates the internal machine *. This supposition 

 in no degree removes the difficulty. Of the princi- 

 ple of life, or that power which enables organized 

 bodies to exhibit living action, we can form some 

 idea, although we know not its nature, just as the 

 astronomer speaks of gravitation without pretending 

 to define what it is ; but of another principle con- 

 nected with life, and producing sensible effects in the 

 system, independently of all the animal functions, 

 we certainly know nothing ; and, fortunately for 

 science, the admission of such imaginary principles, 

 or agents, is now banished from all chaste philoso- 

 phy. Admitting also, with Mr Hunter, the insuffi- 



* Observations on the Animal (Economy, p. 9L 



