172 THE STORY OF THE LIVING MACHINE. 



the uncertainty surrounding this subject, but the 

 almost universal belief in the inheritance of such 

 characters requires us to refer to it again. It is 

 uncertain whether such direct effects have any in- 

 fluence upon the offspring, and therefore whether 

 they have anything to do with this machine build- 

 ing. Still, there are many facts which point 

 strongly in this direction. For example, as we 

 study the history of the horse family we find that 

 an originally five-toed animal began to walk 

 more and more on its middle toe, in such a way 

 that this toe received more and more use, while 

 the outer toes were used less and less. Now that 

 such a habit would produce an effect upon the 

 toes in any generation is evident ; but apparently 

 this influence extended from generation to gen- 

 eration, for, as the history of the animals is 

 followed, it is found that the outer toes became 

 smaller and smaller with the lapse of ages, while 

 the middle one became correspondingly larger, 

 until there was finally produced the horse with its 

 one toe only on each foot. Now here is a line 

 of descent or machine building in the direct line 

 of the effects of use and disuse, and it seems very 

 natural to suppose that the modification has been 

 produced by the direct effect of the use of the 

 organs. There are many other similar instances 

 where the line of machine building has been quite 

 parallel to the effects of use and disuse. If, there- 

 fore, acquired characters can be inherited to any 

 extent, we have, in the direct influences of the 

 environment an important agency in machine 

 building. This direct effect of the conditions 

 is apparently so manifest that one school of 

 biologists finds in it the chief cause of the varia- 

 tions which occur, telling us that the conditions 



