1 8 ELEMENTS OF BIOLOGY. 



It must, however, also be borne in mind that this residuum 

 is probably not to be ascribed to our ignorance, but that it 

 has a real existence. It appears, namely, in the highest 

 degree probable that every vital action has in it something 

 which is not merely physical and chemical, but which is 

 conditioned by an unknown force, higher in its nature and 

 distinct in kind as compared with all other forces. The 

 presence of this '' vital force " may be recognised even in the 

 simplest phenomena of nutrition ; and no attempt even has 

 hitherto been made to explain the phenomena of reproduc- 

 tion by the working of any known physical or chemical 

 force. 



