2 2 BIOLOGY AND HUMAN LIFE 



18. Irritability. We ourselves perceive lights and colors, 

 sounds, odors, and tastes. The movements of the familiar 

 animals show that they are disturbed by much of what happens 

 about them, and these disturbances are different from the kind 

 that is caused when a cup is dropped, for example. A dog does 

 something when he is hurt. Your eye does something when a 

 sudden flash of light is presented. Even a geranium plant 

 changes its behavior when placed in a sunny window. This 

 sensitiveness of living things is in some ways the most remark- 

 able fact about them. 



Yet we shall find that sensitiveness is not altogether confined 

 to living things. There are certain chemical compounds that 

 are in some ways even more sensitive than plants and ani- 

 mals. Some compounds are so sensitive to mechanical disturb- 

 ance that they will produce a violent reaction when they are 

 dropped, as in the case of dynamite, which is by no means the 

 most sensitive. This material is sensitive also to heat. If a 

 hot poker is applied to a stick of dynamite, the results are 

 said to be more disastrous than the consequences of poking a 

 vicious dog. 



19. Fitness. There is one respect, however, in which the sen- 

 sitiveness 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-Hving 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 ." 



