PREFACE. 



The groundwork of the following essay 

 was published in the "Fortnightly Review," for 

 September ist, 1870, in reply to some remarks 

 by Dr. Gull, in his Harveian Oration delivered 

 before the President and Fellows of the Royal 

 College of Physicians, on June 24th of the 

 same year. 



The importance of the issue cannot easily be 

 exaggerated. Forced by the evidence of very 

 many facts to accept the theory of vitality, I 

 would, nevertheless, abandon this idea assailed 

 by Dr. Gull, if the truth of any one of the 

 physical doctrines of life opposed to my views 

 had been rendered probable by scientific evi- 

 dence. So far, however, facts and observations 

 I on things living support the idea of vitality, 

 and are not favourable to any mechanical or 

 chemical hypothesis of life yet proposed, which 



