The linguist, on the other hand, studies the language of all persons alike; the individual features in which the language of a great writer differs from the ordinary speech of his time and place, interest the linguist no more than do the individual features of any other person's speech, and much less than do the features that are common to all speakers.

(1) 如何理解 in which? 为什么要加 in?

【答】in which引导定语从句，先行词为features。介词in表示“方面”，这是因为动词differ的缘故：differ in ... from... 意为“在哪一方面与......不同”。如：

This article differs in writing style from mine. 本文在写作风格上与他的不同。

(2) no more than 和 much less than 后面为什么要加 do？

【答】这是主谓倒装，因为比较状语从句中的两个主语太长，头重脚轻，为了句子匀称而为之。实际为：

no more than the individual features of any other person's speech do

much less than the features that are common to all speakers do

又如：

She sings better than do most of us.  她比我们中大多数人歌唱得好。

The molecules of gases move more freely than do those of liquids and solids.  气体分子运动比液体分子和固体分子运动更为自由。

【翻译】另一方面，语言学家研究的是所有人的语言；一位大作家的语言，其个性特征虽与众不同（同时代和国度），但对语言学家的吸引力并未超过任何其他人，甚至还远逊于大众共有的语言特征（对其吸引的程度）。