170 THE STORY OF LIFE'S MECHANISM. 



duce their effects not by sudden additions to the 

 complication of the machine. They must be 

 constant forces whose effect at any one time is 

 comparatively slight, but whose total effect is 

 to increase 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 

 expect 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 parti- 

 cular 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 

 something of value, but all parts that have been 

 produced are of value at the time of their appear- 

 ance. We must, in short, look for forces con- 

 stantly in action and always tending in the same 

 direction of greater complexity of structure. 



Is it possible to discover these forces and 

 comprehend their action ? Before the modern 

 development of evolution this question would 

 unhesitatingly have been answered in the nega- 

 tive. To-day, under the influence of the descent 

 theory, stimulated, in the first place, by Darwin, 

 the question will be answered by many with 

 equal promptness in the affirmative. At all 



