In the absence of a mark in Sentence 1, a momentary wrong combination is quite unavoidable, because perfectly good sense is made by the wrong grouping. As a comma before “and” may not give to every reader notice of the development of the sentence, and as a semicolon could hardly fail to do so, it would seem to be the better mark.
In neither No. 66-3 nor No. 66-4 can the reader definitely interpret the meaning of the comma until one or two words following the conjunction are reached. As the process of determining a relation from a mark often requires the reader to consider the mark and one or two words following it, which process becomes almost instantaneous, a comma may thus convey its purpose as readily as the semicolon in No. 66-2 tells its purpose.
A slight change in the language of Sentence 1 will not change either the names of the two groups (clauses) or their relation (coördination); but it will remove the liability, even the possibility, of such wrong combination as is suggested in Sentence 1:
66-5. Respect the rights of children and you will gain their respect.
In this sentence, a mark before “and” is not really needed as a warning to the reader not to connect “you” with “children,” thus making “you” an object of the preposition “of.”
The similarity in form between Nos. 1 and 66-5, with a mark of punctuation imperative in the first, suggests the use of a mark in the second. As the comma is here quite sufficient to prevent a wrong combination by the reader, the comma naturally becomes the mark of choice; and good convention confirms this choice, while convention not so good seems to ignore the need of the semicolon in similarly formed sentences in which the apparent and wrong grouping is much more marked than in Sentence 1.
Sentences 66 and 66-5 stand at one extreme of the class of sentences in the punctuation of which we seek to differentiate between the comma and the semicolon. The relation is made quite unmistakable by the shortness and completeness of the first group of words, and also by the fact that the word in each sentence following the conjunction does not suggest, even in the slightest degree, a connection for itself with anything that precedes.
Let us consider a sentence at the other extreme, where a wrong combination is wholly unavoidable without a semicolon, and quite suggestive with one:
66-6. I do not mean that the people are conscious of this fact; but that the leaders of the people are conscious of it, I think, there is no doubt.