The important part of this clause, the only part that adds anything new to the sentence, is the complement, the most desirable kind of genius. The subject which merely represents a previous word, and the verb is is only a link between the subject and its attribute, the complement. These two words help in the grammatical structure of the proposition, but since they are easily supplied by a reader, they may just as well be omitted, as in fact is often the case. This brings the really important words into a position next to what they explain or describe, but without any relation-words between them. A group of words so used is called an appositive phrase. The base-word of the phrase, usually a noun, is called a noun in apposition.
In general the appositive is not a necessary element of the sentence. So far as the truth of the statement and the grammatical structure of the sentence are concerned, it may be omitted. Sometimes, however, it is joined to a noun to point out a special individual or class, when it resembles the restrictive adjective clause; as, “It is quite true that practical life is a kind of long, competitive examination, conducted by that severe pedagogue, Professor Circumstance.”—Huxley.
The Base-Word of an Appositive Phrase.—When the base-word of an appositive phrase is a noun, it gives another name for something, and so makes that thing more fully known by revealing another of its attributes; as, “We turn to commemorate the genius and virtues of John Milton, the poet, the statesman, the philosopher, the glory of English literature, the champion and the martyr of English liberty.”—Macaulay.
Attributes are just as often told by adjectives as by nouns, hence we find many appositive adjectives; as,
“At the first glimpse he thought it a dog—long, lean, skulking, prowling, tawny—on the scent of his tracks.”—J. L. Allen. The adjectives are an abridgment of an unrestrictive adjective clause, just as appositive nouns are.
Besides the noun and the adjective, the base-word of an appositive phrase may be,—
(a) A reflexive pronoun; as, “The old Professor himself sometimes visited the house after it had changed hands.”—Holmes. This pronoun does not explain or describe in the least. It is put into the sentence for emphasis.
(b) A personal pronoun. This forms a convenient word to add a phrase or clause to; as,
“How good it was of him
To mind a slender man like me,