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 that they have a built-in map that guides them for hundreds of miles.
Once they leave the saltwater, they don't eat.
They are determined to finish their journey despite the many obstacles.
prep.不管; 尽管（自己）不愿意; 不在乎; 虽有 ['ɒbstəklz]n.障碍（物）; 障碍物; 绊脚石; 障碍栅栏;
They swim against strong currents, jump over waterfalls and try to escape the claws of hungry bears. 水流 瀑布 爪
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. 生殖，繁殖;
Migration takes animals to new more favorable environments when the seasons change.称赞的; 有利的;
Some of these journeys are long and dangerous, along routes with no obvious landmarks.
路线; 途径 明显的; 陆标
The longest migration is that of the Arctic tern.北极燕鸥;
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.取得 寿命 相当的，等效的
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.移居者; 移民;
Many have died along the way or lived as refugees in camps until they are admitted into a new country.
避难者，难民 营地 许可进入
So the question remains as to how our global community will face these challenges.
社区;
