DIFFERENCES. BETWEEN ANIMALS AND PLANTS. 7 



ence 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 con- 

 venient expression for an unknown quantity, for that residual 

 portion of every vital action which cannot at present be re- 

 ferred to the operation of any known physical 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." 



In the case of the higher animals and plants there is no- 

 difficulty; the former being at once distinguished by the 

 possession of a nervous system, of motor power which can be 

 voluntarily exercised, and of an internal cavity fitted for the 

 reception and digestion of solid food. The higher plants, on 

 the other hand, possess no nervous system or organs of sense, 

 are incapable of independent locomotion, and are not provided 

 with an internal digestive cavity, their food being wholly fluid 

 or gaseous. These distinctions, however, do not hold good as 

 regards the lower and less highly organised members of the 



