举例来看：TPO--The Origins of Cetaceans第一段
It should be obvious that cetaceans—whales, porpoises, and dolphins—are mammals. They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluke1 and blowhole2 cannot disguise their affinities with land dwelling mammals. However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walking mammal and a swimming whale bridged- Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.
这段的第一句为：“It should be obvious that cetaceans—whales, porpoises, and dolphins—are mammals.”显而易见的是，海洋鲸类…都是哺乳动物。这句话显然不是主旨，继续往下读时发现第二句和第三句讲的是鲸类的特点。接着看，主旨句其实是最后一句：“Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.”明显位于陆地哺乳动物和海洋鲸类的中间性过渡性化石直到现在才找到。