THE LIVING MACHINE BUILDING FACTORS. 171 



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 

 generation 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, heredity and varia- 

 tion being simply phases of reproduction. The 

 living machine has developed by -natural pro- 

 cesses, all other machines by artificial methods. 

 Reproduction is the one essential point of differ- 

 ence between the living machine and the others 

 which has made their construction by natural 

 processes a possibility. What, then, is repro- 

 duction 1 Reproduction is in all cases at the 

 bottom simple division. Whether we consider 

 the plant that multiplies by buds or the uni- 

 cellular animal that simply divides into two 

 equal parts, or the larger animal that multi- 



