21. That matters because theory suggests that the maximum sustainable yield that can be cropped from a fishery comes when the biomass of a target species is about 50% of its original levels.

【分析】多重复合句。句子主干为That matters。because引导原因状语从句，其中又包含that引导的从句作suggests的宾语。宾语从句主干为the maximum sustainable yield…comes…，其中又嵌套that引导的定语从句修饰主语yield,另外还包含when引导的时间状语从句。
【译文】这一点很重要，因为理论认为当目标物种的生物量达到原始基数的50%左右时，从渔场中才能够获得最大的可持续产量。
【点拨】crop作名词时，意为“庄稼；收成，产量;一群，一批”。作动词时，意为“收获，获得；剪短”，此外,还有短语crop up“出现或发生（尤指意外地)”。

22. But somewhere from the 19th century onward, more artists began seeing happiness as insipid, phony or, worst of all, boring as we went from Wordsworth's da f fodils to Baudelaire's flowers of evil.

【分析】复合句。句子主干为more artists began seeing happiness as…，其中使用了 see…as结构。句首短语 somewhere from the 19th century onward 作时间状语； as we went from...to...为时间状语从句。
【译文】但到了19世纪的某个时期，更多的艺术家开始将快乐视为索然无味、矫揉造作——甚至更糟的是——令人厌倦的东西。从华兹华斯的《水仙花》到波德莱尔的《恶之花》就可以表明这一变化。
【点拨】l）insipid “枯燥乏味的”。2)phony“虚伪的，虚假的”。