DIFFERENCES BETWEEN ANIMALS AND PLANTS. 13 



produced. What that something is, we do not know, and 

 perhaps never shall know; and it is possible, though highly 

 improbable, that future discoveries may demonstrate that it 

 is merely a subtle modification of some physical force. In 

 the meantime, as all vital actions exhibit this mysterious some- 

 thing, it would appear unphilosophical to ignore its existence 

 altogether, and the term " vital force " may therefore be retained 

 with advantage. In using this term, however, it must not be 

 forgotten that we are simply employing a convenient expres- 

 sion for an unknown quantity, for that residual portion of every 

 vital action which cannot at present be referred to the opera- 

 tion of any known physical force. 



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 reproduction by the working of any 

 known physical or chemical force. 



5. DIFFERENCES BETWEEN ANIMALS AND PLANTS. 



We have now arrived at some definite notion of the essential 

 characters of living beings in general, and we have next to con- 

 sider what are the characteristics of the two great divisions of 

 the organic world. What are the characters which induce us 

 to place any given organism in either the vegetable or the 

 animal kingdom? What, in fact, are the differences between 

 animals and plants? 



It is generally admitted that all bodies which exhibit vital 

 phenomena are capable of being referred to one of the two 

 great kingdoms of organic nature. At the same time it is 

 often extremely difficult in individual cases to come to any 

 decision as to the kingdom to which a given organism should 

 be referred, and in many cases the determination is purely 

 arbitrary. So strongly, in fact, has this difficulty been felt, 

 that some observers have established an intermediate kingdom, 

 a sort of no-man's-land for the reception of those debatable 

 organisms which cannot be definitely and positively classed 



