We punctuate to aid the reader quickly to grasp _through the eye_ the groupings of printed language, and to enable him to determine the relations between the groups thus formed. When a mark is not needed for this purpose, it may be omitted.
If we say _the man is in a bad state of mind and in an equally bad state of body_, the eye catches the words “and” and “in,” at the end of the line, practically at the same instant. The “in” tells of the grouping so distinctly that a comma is not needed to inform the reader that no word follows “and” to be connected by it to “mind.” Therefore we say, when groups are so similarly formed that the word following the conjunction gives ample notice of the grouping, a comma is not needed. We shall consider this point more fully in another place.
By a slight change in the wording of No. 21, we get a counterpart of the illustrative grouping just given:
21-1. He displayed good judgment in his scientific writings and in his public documents.
The grouping here is so unmistakable as to make a comma before “and” quite objectionable. It is unmistakable because of the like formation of the groups, and also because of the absence of any word after “and” that suggests a wrong relation to what precedes.
Now, if we do introduce a word or two between “and” and “in,” and such words do not suggest relation to what precedes, we may still omit the comma before “and.” This reasoning, with that above, suggests the best punctuation of No. 21, which is as follows:
21-2. The deliberate good sense with which Franklin treated matters of religion and morality, he displayed equally in his scientific writings and, a little later, in his public documents and correspondence, which made him as eminent in diplomacy and statecraft as he had earlier been in science and local affairs.
Thus we find no need of any mark before “and” in this sentence, where a painstaking writer uses a semicolon.
As “and,” even in No. 21-1, does not connect “writings” with “documents,” but connects the two groups of words beginning with “in,” the adjective clause in No. 21-2 beginning with “which” can hardly go over into the first group, and there find a noun which it may seem to modify.
To illustrate how puzzling a rule may be, and how wrong, we quote the following rule from Mr. Wilson’s work (page 113):