問：(a) Most of the drugs in current use were discovered by accident or by trail and error, and the cases in which a clear connection has been found between a drug’s action in the body and its chemical and physical properties are few.
(b) You may reply that there is no cause for alarm, because in real situations the total unanimity of a group is rare.
我認為上面兩個句子都犯了頭重腳輕的毛病。
答：在英語裡安排句子成分的次序，主要受兩個原則的支配。一個是尾重原則(the principle of end-weight)，另一個是句尾息核心原則(the principle of end-focus)。
所謂尾重原則，就是把長而複雜的句子的成分放在句尾的原則。這一原則能夠使句子在結構上保持平衡匀稱。下面我們來觀察一下英語是如何實現這一原則的。
A. 根據尾重原則，應把結構較長的副詞放在句末：
He studied them that night with the kind of care his wife had suggested.
B. 按照尾重原則，應把結構複的直接受詞放在句末：
- He had been known to reduce to tears some of the most seasoned and idolized movie-stars in Hollywood.
- She visited that very day an elderly and much beloved friend. (試比較：She visited him that very day.)
- John offered to Mary the help that she needed.
- They pronounced guilty every one of the accused.
C. 按照尾重原則，應把結構複雜的間接受詞放在句末：
I gave a book to each of the boys.
每個男孩我各給了一本書。
D. 按照尾重原則，應當避免主詞部分太重、太長。可通過分割、倒裝、外置等方法來實現這一原則：
- 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) (形容詞 of one in a wild state與其中心詞 photograph被動詞分隔) 然而，事實是我們對大猩猩瞭解得很少。至今還沒有人給處於野生狀態的大猩猩拍攝過一張真正令人滿意的照片…(試比較：No really satisfactory photograph of one in a wild state has ever been taken.)
- Batteries produce electron flow by chemical means–that is, chemical reactions take place inside the battery that result in a flow of electricity. 電池靠化學方法產生電子流—也就是在電池内發生化學反應，形成電流。(試比較chemical reactions that result in a flow of electricity take place inside the battery.）
- A number of cars are already on the road powered by gas turbines. 一些以燃氣渦輪機作動力的汽車已經出現在路上。(試比較：A number of cars powered by gas turbines are already on the road)
- The story is told of her phenomenal success in Australia. 有人講述了她在澳大利亞獲得非凡成功的故事。(試比較：The story of her phenomenal success in Australia is told.)
- Einstein concluded that, ‘In error are those theorists who believe that theory comes inductively from experience’. 愛因斯坦得出結論說：“錯誤的是那些相信理論是由經驗歸納出來的理論家。” (清華大學《新英語教程》第四冊，p.123) (試比較：Those theorists who believe that theory comes inductively from experience are in error.)
- There stood in the way of this conspiracy one great obstacle—Paris. 阻礙這個除謀實現的，有一個巨大的障礙—巴黎。(《法蘭西内戰》,p.42) (試比較：One great obstacle – Paris stood in the way of this conspiracy.)
- It’s certain that we’ll forget the address. 我們肯定會忘記這個地址的。(試比較: That we’ll forget the address is certain)
安排句子成分的次序還要遵循句尾息核心原則(見 end-focus)。這兩個原則在一般情況下並不是互相矛盾的，而是相輔相成的。如果這兩個原則萬一發生了矛盾，則幾乎總是尾重原則服從「句尾訊息核心原則」。例如：
- Of all the worshippers of that day who bent the knee to Smith, first and foremost stood Johnson. 在昔日向史密斯屈膝的所有崇拜者之中，名列第一的是詹森。
- The first person to write a systematic description of elementary geometry was Euclid. 第一個系統地描述初等幾何學的人是歐幾里德。
- Why people are prepared to tolerate a four hour journey each day for the dubious privilege of living in the country is beyond my ken. (New Concept English, Book 3, p.106) 為什麼人們為了享受那頗值得懷疑的鄉間樂趣而寧願忍受每天四小時的來回奔波之苦，這是我所不能理解的。
在你所提問的兩個句子裡，few和rare是訊息重心，只有放在句末才符合句尾訊息核心原則(參見 end-focus)。