He has taken up the aesthetic ideas of Eliot and his successors with anachronistic fidelity. Kirsch is not an academic; most of the essays in “The Modern Element,” his new book on contemporary poetry, first appeared as book reviews in The New Republic. Kirsch writes with admirable clarity for a general reader not automatically familiar with the poets he discusses. But when he is done with his poets, the general reader does not have much reason to read them. Like the poet-critics he admires, Kirsch mounts a defense of poetry at the expense of poetry he disapproves of. His taste tends to be narrow and formulaic; and the results show not only in “The Modern Element,” but in “Invasions,” his new volume of poetry.