听力原文： Good morning, students. I hope you have read the two books about speech. Today's lecture deals with the presence of the unusually large deaf population that existed on the Massachusetts island of Martha's Vineyard for about three centuries. From the settlement of the islands in the 1640s to the 20th century, people there, who were descended from only 25 or 30 original families, married mainly other residents of the island. They formed a highly inbred group, setting an example of the genetic patterns for the inheritance of deafness. Indeed in the late 18th century one out of every 25 people in one village on the island was born deaf, and the island as a whole had a deafness rate at least 17 times greater than that of the rest of the United States. Even Alexander Graham Bell, the inventor of the telephone and a prominent investigator into hearing loss, visited Martha's Vineyard to study the population. But because the principles of genetics and inheritance were still unknown, he was not able to explain the patterns of deafness and why a deaf parent did not always have deaf children. In the 20th century, the local population began to marry people off the island and the rate of deafness started to fall. (33)