Mr Smith had a week's vacation, so he said,“I'm going to the seaside by train. ”He put on his best clothes, took a small bag, went to the railway station(车站)and got into the train. He wore a beautiful hat, and he often put his head out of the window during the trip and looked at the beautiful scenery. Suddenly, the wind pulled his hat off.
     Mr Smith quickly threw(扔) his old bag out of the window, too. The other people in the train laughed,“Is your bag going to bring back your beautiful hat?” they asked.
    “No,” Mr Smith answered.”But My name and address are on the bag. If someone finds both of them, he or she will send me the bag and the hat.” 

人口增长模式由（　）