One or two sentences will illustrate the points under discussion:
36. He cannot understand—nor can any of his leisurely countrymen—why tomorrow will not answer as well as today.
The matter here set off by dashes is a side-remark, purely parenthetical in nature, but given grammatical connection by the use of the conjunction “nor.” If the matter were not purely parenthetical, it would, of course, not take the dashes. A sentence similar in form will illustrate this point:
36-1. He cannot understand, nor does he want to understand, why tomorrow will not answer as well as today.
In the next sentence the grammatical relation is in the nature of apposition; and yet what follows “heroines” is not purely an appositive, for all “female characters” are not “heroines.” The added thought goes beyond the office of parenthetical matter, and becomes an integral part of the sentence, which cannot be omitted without changing the sense:
37. George Eliot’s heroines—her female characters, from first to last—are drawn with the serene firmness of omniscience.
The group of words here set off by dashes expresses too much to be either an appositive of “heroines” or a parenthesis explaining the meaning of “heroines”; in short, it is neither wholly one nor the other, and cannot be properly punctuated as belonging to one or the other class of words.
It may be well to note here the difference between Sentences 36 and 37 and Sentence 32-3, the latter being a type of sentences often improperly punctuated when the sentence is continued beyond the appositive group of words. We have just read such a sentence in the morning newspaper:
38. There is literature in our newspapers—a lively, colloquial, vivid literature, reflective of the life we lead—and we are grateful for his [Augustine Birrell’s] admonition that in the making of it we have regard first for truth and afterward for beauty, if we find it.