LIVING THINGS AND NON-LIVING THINGS 19 



sensitive also to heat ; if a hot poker is apphed to a stick of 

 dynamite, the results are said to be more disastrous than the 

 consequences of poking a vicious dog. 



30. Fitness. There is one respect, however, in which the 

 sensitiveness of living things differs from the sensitiveness of 

 non-living things. In most cases the living body responds to 

 a disturbance by doing something that will probably save it from 

 further injury. The non-living body, when sufficiently disturbed 

 to do anything, does something that generally results in its 

 further injury or destruction. Thus, when a dog's tail is pulled, 

 he will try to run away, or he will bark or snap at the "thing- 

 holding-tail." These responses are, on the whole, of a kind 

 that will save him from further damage. Indeed, we cannot 

 imagine how living beings would continue to live generation 

 after generation if they had the habit of doing things that 

 tended to injure or destroy them. In contrast to this kind of 

 behavior, think of what the stick of dynamite would do if 

 touched with a red-hot poker. There is nothing here that 

 looks in the least like " trying-to-save-itself." 



31. Origin. We do not know anything about the first appear- 

 ance of life upon the earth. But we do know that every plant 

 and animal now living had its origin in the body of some 

 other plant or animal. In general, non-living bodies do not 

 reproduce each other, but, so far as we know, living things can 

 be produced only by other, similar, living things. 



32. Summary. We have seen that growth, movement, and 

 irritability of a certain kind may be present in non-living 

 bodies, but in no case have we found any non-living thing 

 that has all of these properties. Some have one, some another. 

 Living things are characterized by having all three. We rhay 

 say that it is the combination of these properties that distin- 

 guishes living bodies from the non-living and from the dead. 



