THE LIVING MACHINE BUILDING FACTORS. 171 



changes which tend to bring about such migra- 

 tion and isolations, and this factor has doubtless 

 played a more or less important part in the 

 building of the machines. How great a part we 

 cannot say, nor is it necessary for our purpose 

 to decide ; for in all these cases the machine 

 building has only been the result of the heredi- . 

 tary transmission of congenital variation under 

 certain peculiar conditions. The fundamental 

 process is the same as already considered, only 

 the details of its working being in question. 



Direct Influence of the Environment. Under 

 this head we have a subject of great importance. 

 It is an undoubted fact that the environment has 

 a very decided effect upon the machine. These 

 direct effects of the environment are very positive 

 and in great variety. The tropical sun darkens 

 the human skin ; cold climate stunts the growth 

 of plants; lack of food dwarfs all animals and 

 plants, and hundreds of other similar examples 

 could be selected. Another class of similar in- 

 fluences are those produced by use and disuse. 

 Beyond question the use of an organ tends to 

 increase its size, and disuse to decrease it. Com- 

 bats of animals with each other tend to increase 

 their strength, flight from enemies their running 

 powers, etc. 



Now all these effects are direct modifications 

 of the machine, and if they are only transmitted 

 to following generations so as to become perma- 

 nent modifications, they will be most important 

 agencies in the machine building. If, on the 

 other hand, they are not transmitted by heredity, 

 they can have no permanent effect. We have 

 here thus again the problem of the inheritance 

 of acquired characters. We have already noticed 



