THE MECHANICS OF THE LIVING MACHINE 321 



it the processes of life, it becomes evident that the functions 

 of this single machine are as mysterious, although not so com- 

 plex, as are the functions of the whole body of the frog. In 

 other words, getting rid of the complex machinery of such a 

 highly built organism as the frog does not help us at all towards 

 the solution of the* problems of biology; for it is no easier 

 to understand the processes of life going on in the single cell 

 than it is to understand the processes of life going on in the 

 multicellular animal. While the study of single cells and their 

 functions has enabled us to understand the processes 'of life 

 in many respects much better than before, it has not solved 

 the problem of what life is, nor made it any easier to get 

 rid of the idea that living organisms show certain powers 

 not possessed by machines, powers so mysterious that we 

 must acknowledge our inability to explain them, and must, 

 for the present at least, include them under the general term 

 of vitality. 



* The recognition that the cell is such a complex mechanism 

 has recently led to the attempt to analyze it into smaller and 

 simpler units. Whether any success will follow this attempt 

 it is too early to predict. 



For these reasons it is useful still to retain the term "vital 

 force"; not meaning by this to imply that there is any special 

 force in living things, uncorrelated to forces of nature, but 

 simply indicating our present lack of knowledge. By vitality 

 we refer to the guiding principles which regulate the play of 

 chemical and physical forces in this living machine, and which 

 determine the processes of reproduction, which lie at the foun- 

 dation of that side of living organisms and their functions which 

 we call mental. We certainly have not yet explained all the 

 factors connected with life processes, and we can therefore 

 most satisfactorily comprehend them under the term "vitality." 

 With this understanding, it is perfectly legitimate to retain th 3 

 term "vital force" for those phases of life processes which are 

 not included in any mechanical conception of life. 



