THE LIVING ORGANISM 13 



J^ similar to the human process of trial and error which 

 has brought the diverse modern mechanisms to their 

 present conditions of efficiency. This matter, however, 

 must remain for the time just as it stands. The first 

 objection, namely, that an organism ought not to be 

 viewed as a machine, is one that we must meet immedi- 

 ately, because it is necessary at the very outset to gain 

 a clear idea of the essentially mechanical nature of 

 living things and of their relations to the conditions 

 under which they live. It is only when we have such 

 a clear understanding that we can profitably pursue 

 the further inquiries into the evidence of evolution. 

 Our first real task, therefore, is an inquiry into certain 

 fundamental questions about life and living things, upon 

 which we shall build as we proceed. 



All living things possess three general properties which 

 seem to be unique ; these are a peculiar chemical con- 

 stitution, the power of repairing themselves as their 

 tissues wear out, and the abihty to grow and multiply. 

 The third property is so familiar that we fail to see how 

 sharply it distinguishes the creatures of the organic 

 world. To realize this we have only to imagine how 

 strange it would seem if locomotives and steamships 

 detached small portions of themselves which could 

 grow into the full forms of the parent mechanisms. 

 Equally distinctive is the marvelous natural power 

 which enables an animal to re-build its tissues as they 

 are continually used up in the processes of living ; for no 

 man-made, self-sustaining mechanism has ever been 

 perfected. The property of chemical composition is be- 



