自然之谜2
Another amazing example of survival is the salmon.另一个幸存下来的神奇例子是鲑鱼。
Salmon spend most of their lives in the ocean, which is saltwater.鲑鱼大部分生活在海水中。
Yet, when it's time to breed, they do so in freshwater rivers.然而，到了繁殖的时候，它们会在淡水河中繁殖。
Salmon can adapt their bodies to survive in both saltwater and freshwater.鲑鱼可以适应它们的身体，在盐水和淡水中生存。
When it's breeding time, they return to the same river where they were born.到了繁殖的时候，它们回到了它们出生的同一条河。
It seems as if they have a built-in map that guides them for hundreds of miles.好像他们有一张内置的地图，可以指引他们走上几百英里。
Once they leave the saltwater, they don't eat.一旦他们离开盐水，他们就不吃了。
They are determined to finish their journey, despite the many obstacles.尽管有许多障碍，他们决心完成他们的旅程。
They swim against strong currents, jump over waterfalls and try to escape the claws of hungry bears.它们逆流而上，跳过瀑布，试图逃脱饥饿熊的爪。
Many die along the way, but enough of them survive to breed and produce the next generation.许多人一路死去，但他们中有足够的人存活下来繁衍后代。
Once they have laid their eggs, they die, having completed their journey.一旦他们下蛋，他们就死了，完成了他们的旅程。
It seems that their purpose in life is just to reproduce.他们在生活中的目的似乎只是为了繁殖。
Migration takes animals to new, more favorable environments when the seasons change.当季节变化时，迁徙将动物带到新的、更有利的环境中。
Some of these journeys are long and dangerous, along routes with no obvious landmarks.其中一些旅程漫长而危险，沿途没有明显的标志。
The longest migration is that of the arctic tern.最长的迁徙是北极燕鸥。
It flies from pole to pole, twice a year.它一年两次飞跃两极。
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.如果燕鸥的平均寿命达到20年，它飞行的距离就相当于往返月球的旅程。
A landmark is something easy to see and remember, like a river or bay.地标是很容易看到和记住的东西，比如河流或海湾。
Humans also migrate, but seldom if ever for seasonal variations.人类也会迁徙，但很少有季节性变化。
Unfortunately, human migrations are often due to human activities.不幸的是，人类的迁移往往是由于人类的活动。
Since the beginning of the 21st century, large scale migrations have become a global problem.自21世纪初以来，大规模移民已成为一个全球性的问题。
Millions of migrants have had to leave their countries and look for new homes.数以百万计的移民不得不离开他们的国家，寻找新的家园。
Many have died along the way or lived as refugees in camps until they are admitted into a new country.许多人在途中死亡，或作为难民住在难民营，直到他们被接纳到一个新的国家。
So the question remains as to how our global community will face these challenges.因此，问题仍然在于我们的全球社会将如何面对这些挑战。
Most human migrations are due to human activities.大多数人的迁移都是由于人类的活动。
