This realization of the poetry of London is not a small thing.  A city is, properly speaking, more poetic even than a countryside, for while Nature is a chaos of unconscious forces, a city is a chaos of conscious ones.  The crest of the flower or the pattern of the lichen may or may not be significant symbols.  But there is no stone in the street and no brick in the wall that is not actually a deliberate symbol—­a message from some man, as much as if it were a telegram or a post-card.  The narrowest street possesses, in every crook and twist of its intention, the soul of the man who built it, perhaps long in his grave.  Every brick has as human a hieroglyph as if it were a graven brick of Babylon; every slate on the roof is as educational a document as if it were a slate covered with addition and subtraction sums.  Anything which tends, even under the fantastic form of the minutiae of Sherlock Holmes, to assert this romance of detail in civilization, to emphasize this unfathomably human character in flints and tiles, is a good thing.  It is good that the average man should fall into the habit of looking imaginatively at ten men in the street even if it is only on the chance that the eleventh might be a notorious thief.  We may dream, perhaps, that it might be possible to have another and higher romance of London, that men’s souls have stranger adventures than their bodies, and that it would be harder and more exciting to hunt their virtues than to hunt their crimes.  But since our great authors (with the admirable exception of Stevenson) decline to write of that thrilling mood and moment when the eyes of the great city, like the eyes of a cat, begin to flame in the dark, we must give fair credit to the popular literature which, amid a babble of pedantry and preciosity, declines to regard the present as prosaic or the common as commonplace.  Popular art in all ages has been interested in contemporary manners and costume; it dressed the groups around the Crucifixion in the garb of Florentine gentlefolk or Flemish burghers.  In the last century it was the custom for distinguished actors to present Macbeth in a powdered wig and ruffles.  How far we are ourselves in this age from such conviction of the poetry of our own life and manners may easily be conceived by anyone who chooses to imagine a picture of Alfred the Great toasting the cakes dressed in tourist’s knickerbockers, or a performance of ‘Hamlet’ in which the Prince appeared in a frock-coat, with a crape band round his hat.  But this instinct of the age to look back, like Lot’s wife, could not go on for ever.  A rude, popular literature of the romantic possibilities of the modern city was bound to arise.  It has arisen in the popular detective stories, as rough and refreshing as the ballads of Robin Hood.