The first cloning experiments in the 1950s involved frogs and toads. It was not until 1997 that scientists successfully cloned the first mammal, the now world-famous sheep known as Dolly. Cloning techniques have continued to improve since then, and today there are cloned mice, salmon, and cattle. 克隆实验开始于20世纪50年代，对象为青蛙和蟾蜍。直到1997年，科学家才成功地克隆了哺乳类动物，即世界闻名的“多莉羊”。克隆技术从那时起得到了持续的进展，今天已有了克隆老鼠、克隆鲑鱼和克隆牛。 编辑推荐：名师指导公共英语阅读考试技巧 公共英语pets三级全真模拟试卷 公共英语pets二级语法讲解汇总