第2段

Moving from water to land involved a major redesign of every aspect of life, including breathing and reproduction. Nevertheless, a good number of thoroughgoing land animals later turned around, abandoned their hard-earned terrestrial re-tooling, and returned to the water again. Seals have only gone part way back. They show us what the intermediates might have been like, on the way to extreme cases such as whales and dugongs. Whales (including the small whales we call dolphins) and dugongs, with their close cousins the manatees, ceased to be land creatures altogether and reverted to the full marine habits of their remote ancestors. They don’t even come ashore to breed. They do, however, still breathe air, having never developed anything equivalent to the gills of their earlier marine incarnation. Turtles went back to the sea a very long time ago and, like all vertebrate returnees to the water, they breathe air. However, they are, in one respect, less fully given back to the water than whales or dugongs, for turtles still lay their eggs on beaches.

从水生到陆生，涉及到生命的方方面面的重大改造，包括呼吸和繁殖。然而，很多完全离开水域的陆地动物后来又转身回到水中。海豹只是部分回归了水域。它们向我们展示了过渡时期可能是什么样子，这是通向鲸鱼和儒艮等极端案例的途中。鲸鱼（包括我们称之为海豚的小鲸鱼）和儒艮，以及它们的近亲海牛，完全不再是陆地生物，而是回归到了它们远古祖先的完全海洋习性。它们甚至不上岸繁殖。然而，它们仍然呼吸空气，从未发展出与其早期海洋化身的鳃等效的器官。龟类很久以前就回到了海洋，像所有返回水域的脊椎动物一样，它们呼吸空气。然而，在某种程度上，它们对水域的归属不如鲸鱼或儒艮，因为乌龟仍然在海滩上产卵。