194 THE STORY OF LIFE'S MECHANISM. 



peculiarity would no longer give its possessors 

 any advantage over its rivals, since all would 

 possess it. Now, therefore, some new variation 

 would in the same way determine which animals 

 should live and which should die in the struggle, 

 and in time a new modification would be added 

 to the machine. And thus this process continues, 

 one variation after another being added, until 

 the machine is slowly built into a more and 

 more complicated structure, always active but 

 with a constantly increasing efficiency. The 

 construction is a natural one. A mixing of 

 germ plasm in sexual reproduction or some other 

 agencies produce congenital variations; natural 

 selection acting upon the numerous progeny 

 selects the best of the new variations, and 

 heredity preserves and hands them down to 

 posterity. 



All students of whatever school recognize the 

 force of this principle and look upon natural 

 selection as an efficient agency in machine 

 building. It is probably the most fundamental 

 of the external laws that have guided the process. 

 There are, however, certain other laws which 

 have played a more or less subordinate part. 

 The chief of these are the influence of migration 

 and isolation, and the direct influence of the 

 environment. Each of these laws has its own 

 school of advocates, and each has been given 

 by its advocates the chief role in the process 

 of machine building. 



