【问】 (a)Most of the drugs in current use were discovered by accident or by trial and error，andthe cases in which a clear connection has been found between a drug’s action in the bodyand its chemical and physical properties are few.
 (b) You may reply that there is no cause for alarm，because in real situations the total una-nimity of a group is rare.
 我认为上面两个句子都犯了头重脚轻的毛病。
 【答】 在英语里安排句子成分的次序，主要受两个原则的支配。一个是尾重原则(the principleof end-weight)，另一个是句尾信息核心原则(the principle of end-focus)。
 所谓尾重原则，就是把长而复杂的句子的成分放在句尾的原则。这一原则能够使句子在结构上保持平衡匀称。下面我们来观察一下英语是如何实现这一原则的。
 A.根据尾重原则，应把结构较长的状语放在句末：
 He studied them that night with the kind of care his wife had suggested. (CGEL)
 B.按照尾重原则，应把结构复杂的直接宾语放在句末：
 1. He had been known to reduce to tears some of the most seasoned and idolized movie-stars in Hollywood. (CGEL)
 2. She visited that very day an elderly and much beloved friend. (CGEL)(试比较：Shevisited him that very day.)
 3. John offered to Mary the help that she needed. (CGEL)
 4. They pronounced guilty every one of the accused. (CGEL)(正常语序应为SVOC)
 C.按照尾重原则，应把结构复杂的间接宾语放在句末，而打乱了SVOiOd的正常语序：
 I gave a book to each of the boys.(ALD)
 每个男孩我各给了一本书。
 D.按照尾重原则，应当避免主语部分太重、太长。可通过分割、倒装、外置等语法手段来实现这一原则：
 1. Yet the fact is we know very little about gorillas. No really satisfactory photograph
 has ever been taken of one in a wild state... (New Concept English，Book 4)
 (定语 of one in a wild state与其中心词photograph被谓语分隔)
 然而，事实是我们对大猩猩了解得很少。至今还没有人给处于野生状态的大猩猩拍摄过一张真正令人满意的照片……
 (试比较：No really satisfactory photograph of one in a wild state has ever been tak-en.)
 2. Batteries produce electron flow by chemical means—that is，chemical reactions takeplace inside the battery that result in a flow of electricity.
 电池靠化学方法产生电子流——也就是在电池内发生化学反应，形成电流。
 (试比较：chemical reactions that result in a flow of electricity take place inside thebattery.)
 3. A number of cars are already on the road powered by gas turbines.
 一些以燃气轮机作动力的汽车已经出现在路上。
 (试比较：A number of cars powered by gas turbines are already on the road.)
 4. The story is told of her phenomenal success in Australia. (CGEL)
 有人讲述了她在澳大利亚获得非凡成功的故事。
 (试比较：The story of her phenomenal success in Australia is told.)
 5. Einstein concluded that，‘In error are those theorists who believe that theory comesinductively from experience’.
 爱因斯坦得出结论说：“错误的是那些相信理论是由经验归纳出来的理论家。”
 (清华大学《英语教程》，第四册，P.123)
 (试比较：Those theorists who believe that theory comes inductively from experienceare in error.)
 6. There stood in the way of this conspiracy one great obstacle——Paris.
 阻碍这个阴谋实现的，有一个巨大的障碍——巴黎。(《法兰西内战》，P.42)
 (试比较：One great obstacle—Paris stood in the way of this conspiracy.)
 7. It's certain that we'll forget the address.
 我们肯定会忘记这个地址的。
 (试比较：That we'll forget the address is certain.)
 安排句子成分的次序还要遵循句尾信息核心原则(见end-focus)。这两个原则在一般情况下并不是互相矛盾的，而是相辅相成的。如果这两个原则万一发生了矛盾，则几乎总是尾重原则服从句尾信息核心原则。例如：
 1. Of all the worshippers of that day who bent the knee to Smith，first and foremost stoodJohnson.
 在昔日向史密斯屈膝的所有崇拜者之中，名列第一的是约翰逊。
 2. The first person to write a systematic description of elementary geometry was Euclid.第一个系统地描述初等几何学的人是欧几里德。
 3. Why people are prepared to tolerate a four hour journey each day for the dubious privi-lege of living in the country is beyond my ken. (New Concept English，Book 3，P. 106)为什么人们为了享受那颇值得怀疑的乡间乐趣而宁愿忍受每天四小时的来回奔波之苦，这是我所不能理解的。
 在你所提问的两个句子里，few和rare是信息重心，只有放在句末，才符合句尾信息核心原则(参见end-focus)。