THE LIVING MACHINE BUILDING FACTORS. 167 



searching for nature's method of building ma- 

 chines. It is perfectly clear that variations 

 among animals and plants are the foundations of 

 the successive steps in advance made in this 

 machine building, but of course only such varia- 

 tions as can be transmitted to posterity can 

 serve any purpose in this development. If there- 

 fore it should prove that acquired characters can 

 not be inherited, then we should no longer be able 

 to look upon the direct influence of the surround- 

 ings as a factor in the machine building. We 

 should then have nothing left except the congeni- 

 tal variations produced by sexual union, or the 

 direct variation of the germ plasm as a factor for 

 advance. If, however, it shall prove that acquired 

 characters may even occasionally be inherited, 

 then the direct effect of the environment upon the 

 individual will serve as a decided assistance in 

 our problem. 



Here, then, we have before us the factors 

 which have been concerned in the building of the 

 living machine under nature's hands. Reproduc- 

 tion keeps in existence a constantly active, un- 

 stable, readily modified organism as a basis upon 

 which to build. Variation offers constantly new 

 modifications of the type, while heredity insures 

 that the modifications produced in the machine 

 by the influences which give rise to the variations 

 shall be permanently fixed. 



Method of Machine Building. Natural Selec- 

 tion. The method by which these factors have 

 worked together to build up the living machines 

 is easily understood in its general aspects, al- 

 though there are many details as yet unsolved. 

 The general facts connected with the evolution 

 of animals are matters of common knowledge. 



