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.
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.
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.
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.
obstacle 英 [ˈɒbstəkl] 美 [ˈɑːbstəkl] n. 障碍;阻碍;绊脚石;障碍物;障碍栅栏
current 英 [ˈkʌrənt] 美 [ˈkɜːrənt] adj. 现时发生的;当前的;现在的;通用的;流通的;流行的 n. (海洋或江河的)水流;气流;电流;思潮;潮流;趋向
claw 英 [klɔː] 美 [klɔː] n. (动物或禽类的)爪，脚爪;(水生有壳动物的)螯，钳;爪形夹具;(机械的)爪 v. (用爪子或手指甲)抓，撕，挠
favorable 英 [ˈfeɪvərəbl] 美 [ˈfeɪvərəbəl] adj. 赞许的，赞同的;有利的，顺利的;令人愉快的；讨人喜欢的
obvious 英 [ˈɒbviəs] 美 [ˈɑːbviəs] adj. 明显的;显然的;易理解的;公认的;当然的;平淡无奇的;无创意的;因显而易见而不必要的
landmark 英 [ˈlændmɑːk] 美 [ˈlændmɑːrk] n. 陆标，地标(有助于识别所处地点的大建筑物等);(标志重要阶段的)里程碑;有历史意义的建筑物(或遗址)
arctic tern 英 [ˌɑːktɪk ˈtɜːn] 美 [ˌɑːrktɪk ˈtɜːrn] 北极燕鸥
equivalent 英 [ɪˈkwɪvələnt] 美 [ɪˈkwɪvələnt] adj. (价值、数量、意义、重要性等)相等的，相同的 n. 相等的东西;等量;对应词
seldom 英 [ˈseldəm] 美 [ˈseldəm] adv. 不常;很少;难得
seldom if ever 如果有也是少见的，几乎从来不
refugee 英 [ˌrefjuˈdʒiː] 美 [ˌrefjuˈdʒiː] n. 避难者;逃亡者;难民
