SUMMARY 335 



before man could use it. The ancestors of all the horses 

 in America have been brought here, from Europe or Asia, 

 since the days of Columbus. One 'reason why man's 

 progress was far more slow in America than in Europe 

 and Asia may have been that there were no beasts here 

 that could be readily domesticated as helpers of man. 



Man has developed all the different varieties of chickens, dogs, cats, 

 pigeons, etc., as he has developed the varieties of domestic plants; 

 namely, by selecting the individuals that possessed certain qualities 

 that he desired, and then breeding these individuals, generation after 

 generation, so as to make the desired qualities more and more promi- 

 nent. Thus horses are bred for different purposes, such as for racing, 

 for ordinary driving, for hauling loads in the city, and for farm work. 

 The horse that is good for one of these uses is not necessarily good for 

 any of the others. The types of cattle wanted for the commercial 

 production of milk and butter are different from those wanted for 

 meat. Sheep may be bred for the quality of their wool or the quality 

 of their flesh. Breeds of chickens that lay well are often unprofitable 

 as meat producers. The growth of scientific knowledge in the last 

 50 years has greatly increased the efficiency of animal breeding. 



348. Summary. Animals cannot build up their food out of mineral 

 matter ; plants containing chlorophyll can. 



In the ameba a single cell can carry out the necessary animal func- 

 tions. 



The hydra consists of two layers of cells which have slightly differ- 

 ent functions. Each cell can, however, take up all the functions if 

 necessary. 



Starfishes are much more highly developed than hydras. They 

 have the beginnings of a nervous system, a digestive system, a blood 

 system, a breathing system, and of organs of smell and of sight. 



Worms have more symmetry than starfishes, and more highly 

 developed organs. Earthworms are of enormous value to the soil, 

 since they prepare it for plant growth. 



Mollusks include oysters, clams, snails, slugs, and the octopus. 



