196 THE STORY OF LIFE'S MECHANISM. 



tions 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 cli- 

 mate 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 influences are those pro- 

 duced by use and disuse. Beyond question the 

 use of an organ tends to increase its size, and 

 disuse to decrease it. Combats 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, 



