CHAPTER XXI 



THE UNITY OF LIFE 



Questions. 1. How are different parts of the body made to work to- 

 gether ? . 2. Do all plants and animals have nerves ? 3. How can a slight 

 change in one part of an organism bring about suitable responses in other 

 parts ? 4. How can organisms that are so different from each other as 

 the different classes of plants and animals do so many things that are 

 exactly alike ? 



172. Multiplicity of life. There are probably over a million 

 different kinds of plants and over a million different species of 

 animals. No one person can possibly know them all ; but if we 

 consider only those that are familiar to us, it is hard to see what 

 there is about a potato plant and about the potato beetle to 

 make both alive, or why man and the ameba should both be 

 considered animals. Yet throughout this endless variety there 

 are certain facts in common, and it is these that make up lije. 

 In spite of the great variety of form and structure, all life is one 

 in the sense that all organisms, large and small, plant and ani- 

 mal, ancient and modern, all live by doing certain things. They 

 get food, they assimilate it after more or less change, they lib- 

 erate energy by oxidizing assimilated food, and they eliminate 

 wastes. They do other things too, but these they all do, and in 

 fundamentally the same way. 



173. Division of labor. Another problem that we meet when 

 we try to locate life in an organism is the great number of or- 

 gans and processes. In which one of them is life really located? 

 Is it in digesting food or in assimilating it ? in breathing oxygen 

 or in oxidizing ? Is it in the brain, where we are aware of pain and 

 pleasure, of curiosity and fear, of joy and sorrow, or is it in the 

 muscles or flesh, where movement, activity, work, are produced? 



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