THE LIVING MACHINE BUILDING FACTORS. 169 



Each ttlls of its greater and greater differentia- 

 tion of 'labour and structure as the periods of 

 time passed. Each tells of a growing complexity 

 and an increasing perfection of the organisms as 

 successive periods pass. Each tells us of common 

 points of origin and divergence from these points. 

 Each tells us how the more complicated forms 

 have arisen as the results of changes in and 

 modifications of the simpler forms. Each shows 

 us how the individual parts of the organisms 

 have been enlarged or diminished or changed in 

 shape to adapt them to new duties. Each, in 

 short, tells the same story of the gradual con- 

 struction of the living machine by slow steps and 

 through long ages of time. When these three 

 sources of history so accurately agree with each 

 other, it is as impossible to disbelieve in the 

 existence of such history as it is to disbelieve in 

 the existence of the ancient Hittite nation, after 

 its history has been told to us by two different 

 sources of record. 



Now all this is very germane to our subject. 

 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 know- 

 ledge gives us at the very start 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 pro- 



