1 70 THE STORY OF THE LIVING MACHINE. 



chief of these are the influence of migration and 

 isolation, and the direct influence of the environ- 

 ment. Each of these laws has its own school of 

 advocates, and each has been given by its advo- 

 cates the chief role in the process of machine 

 building. 



Migration and Isolation. The production of the 

 various types of machines has been undoubtedly 

 facilitated by the migrations of animals and the 

 isolation of different groups of descendants from 

 each other by various natural barriers. The vari- 

 ations which occur in organisms are so great that 

 they would sometimes run into abnormal struc- 

 tures were it not for the fact that sexual repro- 

 duction constantly tends to reduce them. In an 

 open country where animals and plants interbreed 

 freely, it will commonly happen that individuals 

 with certain peculiarities will mate with others 

 without such peculiarities, and the offspring will 

 therefore inherit the peculiarity not in increased 

 degree but in decreased degree. This constant 

 interbreeding of individuals will tend to prevent 

 the formation of many modifications in the ma- 

 chine which become started by variations. Now 

 plainly if some such individuals, with a peculiar 

 variation, should migrate into a new territory or 

 become isolated from their relatives which do 

 not have similar variations, these individuals will 

 be obliged to breed with each other. The result 

 will be that the next generation, arising thus from 

 two parents each of which shows the same varia- 

 tion, will show it also in equal or increased degree. 

 Migrations and isolations will thus tend to fix in 

 the machine variations which sexual union or 

 other influences inaugurate. Now in the history 

 of the earth's surface there have been many 



