MANUAL OF ZOOLOGY. 



power of manifesting any of the phenomena which constitute 

 what we call life, without losing its hold upon the vital forces 

 which belong to it. 



If, in conclusion, it be asked whether the term " vital force " 

 is any longer permissible in the mouth of a scientific man, 

 the question must, I think, be answered in the affirmative. 

 Formerly, no doubt, the progress of science was retarded and 

 its growth checked by a too exclusive reference of natural 

 phenomena to a so-called vital force. Equally unquestionable 

 is the fact that the development of Biological science has pro- 

 gressed contemporaneously with the successive victories gained 

 by the physicists over the vitalists. Still, no physicist has 

 hitherto succeeded in explaining any fundamental vital phe- 

 nomenon upon purely physical and chemical principles. The 

 simplest vital phenomenon has in it something over and above 

 the merely chemical and physical forces which we can demon- 

 strate in the laboratory. It is easy, for example, to say that 

 the action of the gastric juice is a chemical one, and doubt- 

 less the discovery of this fact was a great step in physiological 

 science. Nevertheless, in spite of the most searching inves- 

 tigations, it is certain that digestion presents phenomena 

 which are as yet inexplicable upon any chemical theory. This 

 is exemplified 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 

 the most highly organised animal with the most complex 

 digestive 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- 



