Yet sensitiveness is not altogether confined to living things. The chemi- 

 cal compounds of the photographic film are in some ways even more sensi- 

 tive than plants and animals. Some compounds are so sensitive that they 

 will produce a violent reaction when they are dropped. It may be more 

 disastrous to push a hot poker into a stick of dynamite than to poke a 

 vicious dog. Unlike a living organism, however, the sensitive dynamite is 

 destroyed by its reaction. 



Fitness If an animal is attacked, it usually acts in a way that will 

 probably save it from further injury. 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". On 

 seeing its kind of food, an animal will usually take steps to get it. Such 

 responses tend, on the whole, to preserve life. This characteristic of plants 

 and animals is sometimes called adaptiveness, or the capacity to fit, more or 

 less completely, the surrounding conditions. Indeed, how could organisms 

 continue to live, generation after generation, if they acted exactly the same 

 under all circumstances? 



Origin We know nothing about the first appearance of life upon the 

 earth. So far as our observation has gone, each plant and animal begins its 

 existence in or on the body of some other plant or animal. In general, or- 

 ganisms reproduce themselves, but nonliving bodies do not. 



Being Alive We may conclude that a living organism, a plant or an 

 animal, is distinguished by these characteristics: It originates from another 

 similar organism. It takes in materials from the outside and assimilates this 

 food into its own substance. It transforms the assimilated material, getting 

 from it the energy by which it moves and carries on other processes. It is 

 sensitive to the conditions and changes in its surroundings. It responds to 

 changes in ways that are adaptive — that is, more or less suited to preserving 

 it, or keepifig it alive. It may reproduce others like itself. 



The adaptiveness of a plant or animal is never perfect. Most living things 

 sufTer injury or privation, and are at last starved or destroyed. Living is a 

 risky business. But even under most favorable conditions, the regular 

 changes which normally take place in a living plant or animal at last come 

 to an end. If not previously "killed", the organism eventually stops living. 

 It dies. Dying is part of life. Nonliving objects can of course be destroyed: 

 but they do not "die". 



What Is there about Plants and Animals That Keeps Them Alive? 



Cells' Plants and animals differ greatly in their forms and in struc- 

 ture and activities; yet they are alike in growing, moving, being irritable, 



iSee No. 4, p. 27. 

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