CLASSES OF ANIMALS 463 



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the rat scatters bits of food that may undergo decay, and dis- 

 tributes the plague and parasitic worms. It sometimes attacks 

 poultry and even larger animals. Of all the mammals the rat 

 should be exterminated first. The problem is partly one of 

 protecting our premises and warehouses from the entry of the 

 rat, and partly one of trapping, poisoning, and starving it. The 

 larger owls are among the best rat exterminators, but they can 

 never meet the situation without our help. 



On the other side of our mammal problem is the fact that 

 many of the native fur-bearing animals are threatened with 

 extinction through excessive trapping and hunting. The game 

 laws of the various states are designed to protect these mammals 

 and to preserve them for the continuous use of human beings. 

 In some parts of the country it will be necessary to do more 

 than pass game laws. Already steps are being taken to estab- 

 lish game preserves in which the animals will have an oppor- 

 tunity to multiply and eventually perhaps to restock other 



regions. 



SUMMARY 



1. There are more branches, or principal divisions, in the animal 

 world than in the plant world. 



2. It is impossible to arrange all animals in a continuous series from 

 lower to higher, but nevertheless some of the branches we can recognize 

 to be higher than others because the individuals have more complex 

 structures, higher degrees of division of labor, and more highly special- 

 ized adaptations of structure to function. 



3. In each branch we can find increasing complexity, division of 

 labor, and specialization of structure. 



4. In all the branches we find the same essential functions performed ; 

 in most branches we can find homologous organs performing different 

 functions. 



5. All animals take part in the general cycle of life whereby the ma- 

 terials of protoplasm are constantly moving on from one organism to 

 another. 



6. In every branch of animals are some of which we can make use, 

 and in every branch are some that are directly or indirectly injurious 

 to us. 



