INTRODUCTION. 



THE Laws of Organic and Animal Life, in re- 

 gard to their susceptibility of demonstration, 

 may be said to possess the properties of the 

 exact and abstract sciences. On many subjects 

 connected with these laws, direct experiments can 

 be brought forward to prove the correctness or 

 fallacy of what is advanced ; but it likewise fre- 

 quently happens that the evidence which is ac- 

 quired, even in this way, must be considered to 

 make only a distant approximation to many im- 

 portant truths. Experiments allow us to go a 

 certain length in endeavouring to develope the 

 different functions of the system ; but unless 

 the reflecting powers enable us to surpass 

 the limits prescribed to our imperfect means 

 and senses, many principles of importance, 

 which are legitimately deduced from general 



