6 MANUAL OP ZOOLOGY. 



structurally little more than a mobile lump of jelly, digests as 

 perfectly as far as the result to itself is concerned as does 

 the most highly organised animal with the most complex di- 

 gestive apparatus. It takes food into its interior, it digests 

 it without the presence of a single organ for the purpose, and 

 still more, it possesses that inexplicable selective power by 

 which it assimilates out of its food such constituents as it 

 needs whilst it rejects the remainder. In the present state of 

 our knowledge, therefore, we must conclude that even in the 

 process of digestion as exhibited in the Amoeba there is some- 

 thing that is not merely physical or chemical. Similarly, any 

 organism when just dead consists of the same protoplasm as 

 before, in the same forms, and with the same arrangement ; 

 but it has most unquestionably lost a something by which all its 

 properties and actions were modified, and some of them were 

 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 meanwhile, as all vital actions exhibit this mysterious 

 something, it would appear unphilosophical to ignore its exist- 

 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 divi- 

 sions 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 diiferences 

 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. 



