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    Some time ago, an old businessman had a large store in Boston. He wanted a boy to work for him, and he put an advertisement in the newspaper. The next morning, many boys came to the old man's office, asking for the job.
    There was a post（柱子）by his desk, and there was a nail（钉子）in it. He took a walking-stick and said, “Everyone has three chances. If anyone can hit the nail on the head with this stick more than once, I will give him the job. ”
    The boy thought it was very easy. They all tried, but nobody could make it. So the old man said, “I'm sorry I can't take any of you. ” And they left.
    He kept the advertisement in the papers, and the next morning many more boys came, and among them he saw a thin boy who had been there the day before. The boy hit the nail successfully for three times. The old man felt surprised and asked how he did that.
    “Yes,” said the boy, “I failed yesterday, but I thought that if I kept on trying a while, I could do it. So I went home and practiced hitting a nail with a stick till I could do it, and I got up very early this morning, and tried again. ”
    The old man said, “You are the boy for me,” and gave him the job. Whatever the boy had to do, he tried to do his best. It was not very long before he became the best clerk in the store.

人类上一代把遗传物质传给下一代是通过（  ）