Allegory meets fable on the fact that both may be satiric; but stands aside from fable on the fact that allegory is much longer and employs personified abstractions as characters. Hawthorne's "Celestial Railroad" is an example of humorous-satiric allegory. Parable, we recall, is always spiritual, allegory often so, and fable never.
Working definition
When you set out to write, therefore, you will have in mind a general summary somewhat like this: An allegory is a narrative of imaginary events designed to teach a series of utilitarian or spiritual truths—the actors in the events being either individuals with typical follies, vices, and virtues, or personified abstractions that go through individual and particular experiences.
How to write allegory
To proceed to write original allegory you will need to pay especial attention to (1) the series of lessons you mean to teach: Shall it be in the realm of politics, trade, education, or general morals? (2) The tone of your teaching: Shall it be humorous or grave? (3) The kind of personages: Shall they be real persons made more-or-less typical and abstract, or shall they be abstractions made more-or-less concrete and individual? (4) The course of the action: What shall happen? There must be something a-doing that is in itself interesting and that has a beginning, a middle, and an end. You must not fall into the error of merely enumerating and cataloguing. You must have a definite action going forward in which your personages take a necessary part. Allegory fell into disrepute in the past because of the attempts of lazy and careless writers. There is evidence of its revival as a popular type. A present-day writer in the Atlantic Monthly has shown us how vigorous, informing, and pungent it may be: "The Novelist's Allegory" is entirely worth while with its good old-fashioned flavor. (5) You must pay attention to the characterizations: you must see to it that the speeches you put into the mouths of your creatures could be delivered by them in the world or society you have got together. Everything in the action—the time, the place, the characters of the persons—must conform to the ideal nature of the subject. The laws of the actual universe you may violate, but not the laws of your imaginary universe. Moreover, the nearer the actual and the imaginary come together on essentials, the more effective your preaching will be. What you write as author's narrative must be vital and contributive.
Make your description of dress and gesture so vivid that it will quicken the imagination of your readers. Never yourself think of your personages as abstractions. Let them live and move before you as real beings; then tell about them.
A testimony to the return of allegory into good favor is its use on the stage. We no longer are afraid to see that Ibsen's "Peer Gynt" is an allegorical satire and not the bucolic love tale that some persons try to make it, and that even the wonderful scene of Ase's death is pathos serving satire. Maeterlinck's "Blue Bird," which is unmistakable allegory, has pleased the latest theater-going public high and low.
Present-day interest in primitive types
This thought leads to a word in general on primitive types. Although it is becoming the fashion to be interested in them, and hence many poor specimens both in prose and verse will get into print, yet the writing of such simple and idealistic things by way of reaction from our intense and often hectic realism, is surely in the main wholesome, regardless of the value of the individual pieces. Years ago Count Tolstoy said, "The artist of the future will understand that to compose a fairy-tale, a little song which will touch, a lullaby or a riddle which will entertain, a jest which will amuse, or to draw a sketch such as will delight dozens of generations or millions of children and adults, is incomparably more important and more fruitful than to compose a novel, or a symphony, or paint a picture of the kind which diverts some members of the wealthy classes for a short time and is then forever forgotten. The region of this art of the simplest feelings accessible to all is enormous, and it is as yet almost untouched." Of course the hope of literary excellence for such an epoch, if it comes, will lie in the possibility of the pieces being kept as Tolstoy's own are, very near to the naïve.