THE LIVING MACHINE BUILDING FACTORS. 195 



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 variations 

 which occur in organisms are so great that they 

 would sometimes run into abnormal structures 

 were it not for the fact that sexual reproduction 

 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 

 machine 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 genera- 

 tion, arising thus from two parents each of 

 which shows the same variation, will show 

 it also in equal or increased degree. Mi- 

 grations 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 

 changes which tend to bring about such migra- 



