THE LIVING MACHINE BUILDING FACTORS. 147 



We are trying to learn how this living machine, 

 with its wonderful capabilities, was built. The 

 history which we have outlined is undoubtedly 

 the history of the building of this machine, and 

 the knowledge that these complicated machines 

 have been produced as the result of slow growth 

 is of the utmost importance to us. This knowl- 

 edge gives us at the very rtart some idea of the 

 nature of the forces which have been at work. It 

 tells us that in searching for these forces we must 

 look for- those which have been acting constantly. 

 We must look for forces which produce their 

 effects not by sudden additions to the complica- 

 tion of the machine. They must be constant 

 forces whose effect at any one time is com- 

 paratively slight, but whose total effect is to in- 

 crease the complexity of the machine. They 

 must be forces which produce new types through 

 the modification of the old ones. We must look 

 for forces which do not adapt the machine for its 

 future, but only for its present need. Each step 

 in the history has been a complete animal with its 

 own fully developed powers. We are not to ex- 

 pect to find forces which planned the perfect 

 machine from the start, nor forces which were 

 engaged in constructing parts for future use. 

 Each step in the building of the machine was 

 taken for the good of the machine at the particu- 

 lar moment, and the forces which we are to look 

 for must therefore be only such as can adapt the 

 organisms for its present needs. In other words, 

 nothing has been produced in this machine for 

 the purpose of being developed later into some- 

 thing of value, but all parts that have been pro- 

 duced are of value at the time of their appearance. 

 We must, in short, look for forces constantly in 



