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 hundred of miles.
(Where do selmons spent most of their lives?) in the ocean
(why do salmon return to the same river where they were born?) to breed
(what did they do after laying their eggs?)
salmon n.鲑; 大麻哈鱼;
survival n.生存; 存活; 幸存; 残存物; 幸存事物;
saltwater n.海水; 咸水; 盐水;
life n.生命; 人命; 性命; 人的存活; 生物; 活物; 复数是lives
breed v.交配繁殖; 饲养，培育(动植物); 孕育; 导致;
breeding n.(为繁殖的) 饲养; (动植物的) 生育，繁殖; 教养;
freshwater adj.淡水中生长的; 淡水的;
adapt to 适应;
built-in adj.是…的组成部分的; 嵌入式的; 内置的;
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 eacape 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.
(What did they do after laying their eggs?)
Once conj.一…就; 一旦; 当…时候;
conj 连词; 复共轭; 任意一个;
are determined to 决定; 下决心; determined adj.决心; 决定; 决意; 坚定的; 坚决的; 果断的;
obstacle n.障碍; 阻碍; 绊脚石; 障碍物; 障碍栅栏;
current n.(海洋或江河的) 水流; 气流; 电流; 思潮; 潮流; 趋向;
jump over （使） 跳过;
waterfall n.瀑布;
claw 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;
once have done 一旦你已经。。。，表明动作已经完成，而且要对以后产生影响，用现在完成时来表示。
lay egg 产卵; 下蛋;
lay v.(尤指轻轻地或小心地) 放置，安放，搁; 铺，铺放，铺设(尤指在地板上)
having done sth 一般出现在句首,表达出“在...的前提下”那层意思,后面必定跟着一条完整从句.
reproduce v.复制; 再生产; 再制造; 使再次发生; 再现; 繁殖; 生育;
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. that指代的是migration
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.
(A landmark is something easy to see and remarker, like a river or bay)
migration n.迁移; 移居; 迁徙; (计算机系统的) 改变; (程序或硬件的) 迁移，转移;
favorable adj.赞许的，赞同的; 有利的，顺利的; 令人愉快的；讨人喜欢的;
seasons change 网络季节变化;
route n.路线; 路途; (公共汽车和列车等的) 常规路线，固定线路; 途径; 渠道;
landmark 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 北极燕鸥
arctic n.北极（区）；北极圈; （常复数） 御寒防水套鞋 adj.北极的; 严寒的，极冷的;
tern n.燕鸥;
pole n.柱子; 杆子; 棍; 杖; (行星的) 极; 地极; 磁极; 电极;
achieve v.(凭长期努力)达到(某目标、地位、标准) ; 完成; 成功;
average lifespan 网络平均寿命;
lifespan n.寿命; 可持续年限; 有效期;
equivalent to 网络相当于; 等于; 相等的; 相等於; 等同于;
equivalent 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;
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 remians as to how our global community will face these challenges.
seldom if ever 如果有也是少见的，几乎从来不;
since prep.自…以后; 从…以来; (表示气愤) 何曾，什么时候; 介词
conj.从…以后; 自…以来; 因为; 由于; 既然; 连词
adv.自…以后; 从…以来; 此后; 后来; 副词
migrant n.(为工作) 移居者; 移民; 候鸟; 迁徙动物;
refugee n.避难者; 逃亡者; 难民;
remian v.仍然是; 保持不变; 剩余; 遗留; 继续存在; 仍需去做(或说、处理);
as to 至于，谈到; 关于; 成比例;
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