CHAPTER XLI 



CLASSES OF ANIMALS AND THEIR ECONOMIC IMPORTANCE 



Questions. 1. How can we tell what animals are useful and what ones 

 are harmful ? 2. Could we get along without animals ? 3. Are there 

 common names in English for all kinds of animals ? 4. Are there com- 

 mon names for all the families, classes, orders, and main divisions ? 

 5. Why do we have Latin names for different animals ? 6. Why do we 

 consider one class of animals higher than another ? 7. Why are not ani- 

 mals always able to protect themselves from their natural enemies ? 

 8. Why are not animals always able to get the food that they naturally 

 need ? 9. Is it possible to exterminate any species of animal ? Is it ever 

 desirable to do so? Is it right to do so? 



326. General relations. With animals as with plants a par- 

 ticular species may be related to us not because of any peculiar- 

 ity of structure, habit, or chemical properties but because of the 

 general fact that it is a living thing and so a link in the complex 

 chain of interchange of materials (see Fig. 176). The protozoa, 

 for example, are too small for us to eat, but they are not too 

 small to serve as food for microscopic crustaceans, which in turn 

 are devoured by larger animals, and these by still larger, until 

 we come to fish that can serve us as food. ]\Iany animals, like 

 many plants, serve in this cycle of life in the role of scavengers ; 

 that is, they feed upon the dead remains of other organisms and 

 thus make dead matter at last available to man's purposes. 

 This does not mean that every animal is of some use to mankind, 

 either directly or indirectly, for that is not true of animals any 

 more than it is of plants. Like plants, animals may be of in- 

 direct as well as of direct injury to us, to a very serious degree. 

 A large part of the work which human beings do every year for 

 the purpose of raising plants and animals is destroyed or spoiled 

 by insects, by rats, and by other animals. 



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