134 THE STORY OF LIFE'S MECHANISM. 



earlier notion of protoplasm as the physical basis 

 of life. There can be no life without the pro- 

 perties of assimilation, growth, and reproduction ; 

 and, so far as we know, these properties are 

 found only in that combination of bodies which 

 we call the cell, with its mixture of harmoniously 

 acting parts. Life, at least the life of a cell, is 

 then not the property of a chemical compound proto- 

 plasm, but is the result of the activities of a machine. 

 Indeed, we are now at a loss to know how we can 

 retain the term protoplasm. As originally used it 

 meant the contents of the cell, and the significance 

 in the term was in the conception of protoplasm 

 as a somewhat homogeneous chemical compound 

 uniform in all types of life. But we now see 

 that this cell contains not a single substance, but 

 a large number, including solids, jelly masses, 

 and liquids, each of which has its own chemical 

 composition. The number of chemical com- 

 pounds existing in the material formerly called 

 protoplasm no one knows, but we do know that 

 they are many, and that the different substances 

 are combined to form a physical structure. 

 Which of these various bodies shall we continue 

 to call protoplasm ? Shall it be the linin, or the 

 liquids, or the microsomes, or the chromatin 

 threads, or the centrosomes ? Which of these is 

 the actual physical basis of life ? From the de- 

 scription of cell life which we have given, it will 

 be evident that no one of them is a material upon 

 which our chemical biologists can longer found a 

 chemical theory of life. That chemical theory of 

 life, as we have seen, was founded upon the con- 

 ception that the primitive life substance is a 



