It follows from this that what remains in the world of observation in personality or experience, whether good or evil, whether particular or general, not yet coordinated in rational knowledge as a whole, all for which no solution is found, all that cannot be or has not been made intelligible, must be the subject-matter of realism in the exact use of that term.  This must be recorded by literature, or admitted into it, as matter-of-fact which is to the mind still a problem.  Earthly mystery therefore is the special sphere of realism.  The borderland of the unknown or the irreducible is its realm.  This old residuum, this new material, is not yet capable of art.  Hence, too, realism in this sense characterizes ages of expansion of knowledge such as ours.  The new information which is the fruit of our wide travel, of our research into the past, has enlarged the problem of man’s life by showing us both primitive and historical humanity in its changeful phases of progress working out the beast; and this new interest has been reenforced by the attention paid, under influences of democracy and philanthropy, to the lower and baser forms of life in the masses under civilization, which has been a new revelation of persistent savagery in our midst.  Here realism illustrates its service as a gatherer of knowledge which may hereafter be reduced to orderliness by idealistic processes, for idealism is the organizer of all knowledge.  But apart from this incoming of facts, or of laws not yet harmonized in the whole body of law, for which we may have fair hope that a synthesis will be found, there remains forever that residuum of which I spoke, which has resisted the intelligence of man, age after age, from the first throb of feeling, the first ray of thought; that involuntary evil, that unmerited suffering, that impotent pain,—­the human debris of the social process,—­which is a challenge to the power of God, and a cry to the heart of man that broods over it in vain, yet cannot choose but hear.  In this region the near affinity of realism to pessimism, to atheism, is plain enough; its necessary dealing with the base, the brutal, the unredeemed, the hopeless darkness of the infamies of heredity, criminal education, and successful malignity, eating into the being as well as controlling the fortune of their victims, is manifest; and what answer has ever been found to the interrogation they make?  It is not merely that particular facts are here irreconcilable; but laws themselves are discernible, types even not of narrow application, which have not been brought into any relation with what I have named the divine order.  Millions of men in thousands of years are included in this holocaust of past time,—­eras of savagery, Assyrian civilizations, Christian butcheries, the Czar yet supreme, the Turk yet alive.