148 THE STORY OF THE LIVING MACHINE. 



action and always tending in the same direction 

 of greater complexity of structure. 



Is it possible to discover these forces and com- 

 prehend their action ? Before the modern de- 

 velopment of evolution this question would un- 

 hesitatingly have been answered in the negative. 

 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 events, we 

 have learned in the last forty years to recognize 

 some of the factors which have been at work in 

 the construction of this machine. We must turn, 

 therefore, to the consideration of these factors. 



Forces at Work in the Building of the Living 

 Machine. There are three primary factors which 

 lie at the bottom of the whole process. They 

 are 



1. Reproduction, which preserves type from gen- 

 eration to generation. 



2. Variation, which modifies type from genera- 

 tion to generation. 



3. Heredity, which transmits characters from 

 generation to generation. 



Each must be considered by itself. 



Reproduction. Reproduction is the primary 

 factor in this process of machine building, hered- 

 ity and variation being simply phases of repro- 

 duction. The living machine has developed by 

 natural processes, all other machines by artificial 

 methods. Reproduction is the one essential point 

 of difference between the living machine and the 

 others which has made their construction by 

 natural processes a possibility. What, then, is 

 reproduction ? Reproduction is in all cases at 

 the bottom simple division. Whether we consider 



