18．1原因法
有时，一个结果会源于几个原因。同样，段落可或以结果开头或以结果结束，作者可以从熟悉到不熟悉的原因逐一分析，或是从分析一个又一个读者不熟悉的原因开头，最后引出读者熟悉的结果。选用哪一种，决定于作者认为哪一种更有说服力。
示范段落18-1
Several changes have brought wild animals to the cities．Foremost is，that air and water quality in many cities have improved as a result of the 1970's pollution control efforts．Meanwhile，rural areas have been built up，leaving many animals on the edges of suburbs．In addition，urban wildlife refuges have been created．The Greater London Council last year spent ＄750,000 to buy land and build 10 permanent wildlife refuges in the city．As a result，many big birds are now living in the city．For peregrine falcons cities are actully safer than rural cliff dwellings．By 1970 the birds had died out east of the Mississippi because DDT had made their eggs too thin to support life．That year，scientist Tom Cade of Cornell University began raising the birds for release in cities，for cities afforded plentyof food．
18．2 示范段落分析
主题句：Several changes have brought wild animals to the cities．
因果模式（Cause-effect Pattern）：
结果（Effect）：wild animals back to the cities
原因（Causes）： 1．Air and water quality have improved．
2．Rural areas have been built up．
3．Urban wildlife refuges have been created．
18．3 因-果-因法
有时候，一个原因也许会引起一个结果，这个结果又成为另一个结果的原因，如示范段落18-2所示。
示范段落18-2
For some time now，medical scientists have noted an alarming increase in disease of the heart and circulation among people who smoke cigarettes．It has been found that the presence of nicotine in the blood stream causes blood vessels to contract，thus slowing circulation，which eventully leads to hardening of the arteries．As the arteries stiffen，less blood reaches the brain，and the end result of this slowdown is the cerebral hemorrhage，commonly referred to as a"stroke"．In addition，nicotine in the bloodstream reduces the ability of the hemoglobin to release oxygen，resulting in shortness of breath．The lack of oxygen forces the heart to beat faster-that is，the pulse rate increases-and in turn accelerates the risk of heart attack．
18．4．示范段落分析
因-果-因模式（Cause-Effect-Cause Pattern）：
Smoking tobacco→nicotine in the blood streams
→the blood vessels constract...
18．5 关于写因果段落的建议（续）
3）着重分析直接的、明显的原因
每个结果都有无数的原因，同样，每个行为都会导致无数的结果。因果段落应只着重分析直接的、明显的原因，忽略间接的原因。例如，一个学生成绩不好，直接原因是没有认真学习，间接原因也许是对学校不满。显然，我们应该着眼于直接的原因而摒弃间接的原因。
4）根据重要性或熟悉程度排列细节（见第19单元）
5）使用适当的排列符号（listing signals）（见第34单元）
Exercise 18-1
Directions：Read the following paragraph of cause and effect，adding suitable listing signals and linking expressions．
There are many reasons why languages change．____，
various languages that started from the same parent developed their own uniqueness after groups of speakers drifted away from one another to establish isolated，independent communities．________is the independence of and interaction with foreign cultures，often as a result of military conquest．________is rapidly expanding technologe and new systems of communication that bring all cultures and languages into close contact，with borrowing between languages a common phenomenon in the contemporary world．All languages change as the experiences of their speakers change．
Exercise 18-2
Directions：Write a paragraph analyzing the causes of students cheating on exams．
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