Whales—or their ancestors—were once land animals. Scientists believe that they roamed① the earth about 150million years ago.
The land welling ancestors of this modern king of the sea were hunters② with legs and the jaws③ and teeth. Their favorite hunting grounds were probably shallow④ waters near the mouths of rivers or off a level stretch of coast， for both finny⑤ fish and shell were then more plentiful and easier to catch than animals. Because of this， these land mammals came to spend more and more time in the water.
With ages passing， and these creatures swam more easily than they walked；millions of years later they were able to do without legs altogether. They were now recognizable as whales. Their forelegs had turned into the flippers⑥ that today’s whales use for steering；their hind⑦ legs had shrunk⑧ so that mere traces of them can be found under the skin when a whale is dissected⑨.
① roamv.漫步，漫游，流浪
② huntern.猎人，猎手
③ jawn.颌，颚[C]
④ shallowadj.浅的
⑤ finnyadj.有鳍的，鳍状的
⑥ flippern.鳍状肢
⑦ hindadj.后面的，后部的，在后的
⑧ shrunkv.(shrink的过去式和过去分词)收缩，缩短
⑨ dissectv.解剖，切开
从陆上动物到海中之王
鲸——或者说鲸的祖先——曾经是陆地上的动物。科学家们认为，大约在1.5亿年以前，它们曾在陆地上转悠。
这个现代海中之王的陆上祖先，是有腿、有颚、有利齿的猎手。它们最喜欢的狩猎场，是靠近河口或是平坦海岸的近海浅水区，因为当时有鳍的鱼类和贝壳类动物比现在要多，而且比其他动物更容易捕捉。由于这一缘故，这些陆地上的哺乳动物便逐渐在水中呆得越来越长了。
随着时代的变迁，这些动物在水里游起来比在陆地上行走更加悠然自得；几百万年以后，它们便压根儿不需要腿了，这时就可辨认出它们是鲸了。它们的前腿变成鳍状肢，今天的鲸鱼便是用它来掌握方向的；它们的后腿萎缩了，因而只有当鲸鱼被解剖时，才能在皮肤下见到后腿的一些痕迹。