12 MANUAL OF ZOOLOGY. 



capable of correlation with the ordinary physical or chemical 

 forces. It cannot, however, be said that this view has as yet 

 received a scientific demonstration. On the other hand, it 

 seems safer, with our present knowledge, to believe that proto- 

 plasm is simply the necessary material basis or vehicle through 

 which vital force is manifested, though we are still unable to 

 speak with any positiveness as to the precise nature of the 

 forces which are the fundamental causes of life. 



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, in the meanwhile, be answered in the affirma- 

 tive. 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 ex- 

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

 organism like the Amoeba. This animalcule, which is struc- 

 turally little more than a mobile lump of semi-fluid protoplasm, 

 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 



