MANUAL OF ZOOLOGY. 



retained with advantage. In using this term, however, it 

 must not be forgotten that we are simply employing a con- 

 venient expression for an unknown quantity, for that residual 

 portion of every vital action which cannot at present be 

 referred to the operation 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 phe- 

 nomena of nutrition ; and no attempt even has hitherto been 

 made to explain the phenomena of reproduction by the work- 

 ing of any known physical or chemical force. 



4. DIFFERENCES BETWEEN ANIMALS AND PLANTS. 



We have now arrived at some definite notion of the essen- 

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

 consider 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 

 either amongst vegetables or amongst animals. Thus, Dr 

 Ernst Haeckel has proposed to form an intermediate kingdom, 

 which he calls the Regnum Protisticum, for the reception of all 

 doubtful organisms. Even such a cautious observer as Dr 

 Rolleston, whilst questioning the propriety of this step, is 

 forced to conclude that " there are organisms which at one 

 period of their life exhibit an aggregate of phenomena such as 

 to justify us in speaking of them as animals, whilst at another 

 they appear to be as distinctly vegetable." 



