Another amazing example of survival is the salmon. Salmon spend most of their lives in the ocean, which is saltwater. Yet, when it's time to breed, they do so in freshwater rivers. Salmon can adapt their bodies to survive in both saltwater and freshwater. When it's breeding time, they return to the same river where they were born. It seems as if they have a built-in map that guides them for hundreds of miles.
另一个神奇的生存例子是鲑鱼。鲑鱼大部分的生命时光在海洋中度过，海洋是咸水。然而，当繁殖的时候，它们会去淡水河流中繁殖。鲑鱼能让它们的身体适应咸水和淡水。到了繁殖期，它们会回到当初出生的河流。它们似乎有一张内置的地图，可以指引它们游上数百英里。

Once they leave the saltwater, they don't eat. They are determined to finish their journey, despite the many obstacles. They swim against strong currents, jump over waterfalls and try to escape the claws of hungry bears. Many die along the way, but enough of them survive to breed and produce the next generation. Once they have laid their eggs, they die, having completed their journey. It seems that their purpose in life is just to reproduce.
一旦离开咸水，它们就不再吃东西。尽管一路坎坷，它们还是决心要完成它们的旅程。它们逆流而上，跃过瀑布，逃脱饥肠辘辘的狗熊的爪子。许多同伴在途中死去，但它们中有足够的数量存活下来繁育下一代。一旦产完卵，它们就死了，完成了它们的旅程。似乎它们生活的目的就是繁育后代。

Migration takes animals to new, more favorable environments when the seasons change. Some of these journeys are long and dangerous, along routes with no obvious landmarks. The longest migration is that of the Arctic tern. It flies from pole to pole, twice a year. If a tern achieves its average lifespan of 20 years, the distance it has flown will be equivalent to a journey to the moon and back.
当季节变化时，迁徙把动物们带到新的、更有利的环境中。有些旅程漫长而危险，沿途没有明显的地标。最长的迁徙是北极燕鸥。它从一个极点飞到另一个极点，一年两次。如果燕鸥的平均寿命为20年，那么它飞行的距离相当于一次往返月球。

Humans also migrate, but seldom if ever for seasonal variations. Unfortunately, human migrations are often due to human activities. Since the beginning of the 21st century, large scale migrations have become a global problem. Millions of migrants have had to leave their countries and look for new homes. Many have died along the way or lived as refugees in camps until they are admitted into a new country. So the question remains as to how our global community will face these challenges.
人类也会迁徙，但很少会因为季节变化而迁徙。不幸的是，人类迁徙往往是由于人类活动。21世纪以来，大规模的移民已经成为一个全球性的问题。数以百万计的移民不得不离开他们的国家，去寻找新的家园。许多人死在路上，或者生活在难民营，直到进入一个新的国家。因此，我们的全球社会将如何面对这些挑战仍是一个问题。