The Century gives a definition that is based upon the Greek roots of the word _parenthesis_, as well as upon its use by good writers. According to this definition, a parenthesis has _no grammatical_ relation with the language into which it is “put.” (The word _thrust_ would perhaps give a clearer idea of the original.) We “parenthesize” in spoken, as well as in written or printed, language; but in the former we convey the meaning by inflections and pauses, while in the latter we convey the meaning by marks of punctuation.
The use of the conjunction (and) in the sentence under consideration clearly takes the inserted group of words out of the proper definition of a parenthesis; for it puts such words into _grammatical_ connection with what precedes, although the words are quite strongly parenthetical in nature.
Now, having eliminated the comma, the semicolon, and the parentheses as the proper marks for setting off this group of words _thrust_ into the sentence, and still making good sense by their grammatical connection, how shall we punctuate them? It is perfectly evident that this group of words makes a decided _break_ in the thread of the thought, and that they do not have logical or natural connection with what precedes, because the true _and_ relation does not exist between the thoughts, implied or expressed. Such language may be said to be idiomatic and therefore proper; but the group of words is not properly set off in either No. 67 or No. 67-1. As the fundamental use of the dash is to note such a break in language as here occurs, it is the proper mark before “and” in this sentence. In language of this kind it may be said that the writer _dashes_ off the track of his thought for a moment; and therefore the dash is the proper mark to indicate such a break in the continuity of thought.
The second dash is used to close the group in the same way that the second comma of a pair of commas, or the second part of the marks of parenthesis, is used; and this makes the punctuation of No. 67-2 correct punctuation, for it is based upon the fundamental meanings of both the marks and the sense relations of the words.
This careless use of the marks of parenthesis, especially as in No. 67-1, is quite common in some literary periodicals of high standing and in not a few large printing establishments; but, we venture to assert, it is rare among painstaking writers who punctuate their own writings.