The poetry of Burroughs faded more and more from his work with every book, and the spirit of the scientist, of the trained observer impatient of everything not demonstrable by the senses, grew upon him, until at length it took full control and expressed itself as criticism, as scientific controversy, and as philosophical discussion. _Riverby_, 1894, with its prefatory note stating that the volume was "probably my last collection of out-of-door papers," marks the point of division between the two periods. If we follow the Riverside edition, at present [1914] the definitive canon, eight books preceded _Riverby_ and eight followed it. The groups are not homogeneous; it is not to be gathered that on a certain date Burroughs abandoned one form of essay and devoted himself exclusively to another, but it is true that the work of his last period is prevailingly scientific and critical. His _Indoor Studies_, 1889, _Whitman, a Study, and Literary Values_ are as distinctively works of literary criticism as Arnold's _Essays in Criticism_; his _Light of Day_ discusses religion from the standpoint of the scientist; his _Ways of Nature_ is scientific controversy; and his _Time and Change_ and _The Summit of the Years_ are philosophy. It is in this second period that Burroughs has done his most distinctive work, though not perhaps his most spontaneous and delightful. By temperament and training he is a critic, a scientific critic, an analyzer and comparer. Only men of positive character, original forces, attract him: Emerson and Whitman, and later Wordsworth, Carlyle, and Arnold, men who molded the intellectual life of their age. His first published book had been a critical study, _Notes on Walt Whitman_, 1867, a work the most wonderful in many ways of his whole output. It came at a critical moment, in those pregnant closing years of the sixties, and it struck clear and full the note of the new period. Burroughs's later studies of Whitman are more finished and more mature than this never-republished volume, but they lack its clarion quality. It is more than a defense and an explanation of Whitman: it is a call to higher levels in literature and art, a call for a new definition of poetry, a condemnation of that softness and honey sweetness of song that had lured to weakness poets like Taylor and Stoddard. Poetry henceforth must be more than mere beauty for beauty's sake: it must have a message; it must come burning from a man's soul; it must thrill with human life.