USE OF THE TERM VITAL FORCE. 1 7 



It is easy, for example, to say that the action of the gastric 

 juice is a chemical one, and doubtless the discovery of this 

 fact was a great step in physiological science. Neverthe- 

 less, in spite of the most searching investigations, it is cer- 

 tain that digestion presents phenomena which are as yet 

 inexplicable upon any chemical theory. This is excmphficd 

 in its most striking form, when we look at a simple organism 

 like the Amoeba. This animalcule, which is structurally 

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

 — as far as the result to itself is concerned — as does tlie 

 most highly organised animal with the most complex diges- 

 tive 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 

 something* that is i]Ot merely physical or chemical. Simi- 

 larly, 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 some- 

 thing 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 unphilo- 

 sophical 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 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. 



