THE CELL AND PROTOPLASM. 117 



Bits of iron and steel, for instance, are put to- 

 gether to form a locomotive, but the action of 

 the locomotive depends, not upon the chemical 

 forces which made the steel, but upon the rela- 

 tion of the bits of steel to each other in the ma- 

 chine. So far as we have had any experience, 

 machines have been built under the guidance of 

 intelligence which adapts the parts to each other. 

 When therefore we find that the simplest life sub- 

 stance is a machine, we are forced to ask what 

 forces exist in nature which can in a similar way 

 build machines by the adjustment of parts to each 

 other. But this topic belongs to the second part 

 of our subject, and must be for the present post- 

 poned. 



Reaction against the Cell Doctrine. As the 

 knowledge of cells which we have outlined was 

 slowly acquired, the conception of the cell passed 

 through various modifications. At first the cell 

 wall was looked upon as the fundamental part, 

 but this idea soon gave place to the belief that it 

 was the protoplasm that was alive. Under the in- 

 fluence of this thought the cell doctrine developed 

 into something like the following: The cell is 

 simply a bit of protoplasm and is the unit of liv- 

 ing matter. The bodies of all larger animals and 

 plants are made up of great numbers of these 

 units acting together, and the activities of the 

 entire organism are simply the sum of the activi- 

 ties of its cells. The organism is thus simply the 

 sum of the cells which compose it, and its activi- 

 ties the sum of the activities of the individual 

 cells. As more facts were disclosed the idea 

 changed slightly. The importance of the nucleus 

 became more and more forcibly impressed upon 

 microscopists, and this body came after a little 



