The organic form, as opposed to the mechanic, is a form that is dictated by its specific content and not by a mechanic or pre-determined system.
From A Poet’s Glossary
The following definition of the term organic form is reprinted from A Poet's Glossary by Edward Hirsch.
Since the development of natural history and biology in the eighteenth century, the word organic has primarily referred to things living and growing. Machines took on new significance during the Industrial Revolution, and romantic thinkers began to reject eighteenth-century mechanical philosophies of mind, differentiating between organic and inorganic systems, natural and mechanical bodies. Taking a lead from the German critic A. W. Schlegel, Samuel Taylor Coleridge distinguished between mechanic form and organic form in an essay on Shakespeare:
The form is mechanic when on any given material we impress a pre-determined form, not necessarily arising out of the properties of the material — as when to a mass of wet clay we give whatever shape we wish it to retain when hardened. The organic form on the other hand is innate, it shapes as it develops itself from within, and the fullness of its development is one and the same with the perfection of its outward Form. Such is the Life, such is the form. Nature, the prime genial artist, inexhaustible in diverse powers, is equally inexhaustible in forms.
Coleridge made a strong distinction between the mechanical fancy and the living imagination, and suggested that the work of art is like a living organism, especially a plant, which originates in a seed, continues to grow (in Shakespeare, “All is growth, evolution, genesis,—each line, each word almost, begets the following”), assimilates and “enters into open communion with all the elements,” and evolves spontaneously from within,” effectuating “its own secret growth.”
The metaphor of organic or appropriate form, something that develops naturally from within, has been crucial to the development of romantic and certain crucial strands of American poetry. The idea that art derives from nature rather than from other art has fueled American ideas of originality. Ralph Waldo Emerson created a credo for American poetry when he adapted Coleridge’s botanical metaphor for poetic form and declared in “The Poet” (1844): “For it is not metres, but a metre-making argument, that makes a poem,—a thought so passionate and alive, that, like the spirit of a plant or an animal, it has an architecture of its own, and adorns nature with a new thing.” Henry David Thoreau similarly used the language of biology for the genesis of poems: “As naturally as the oak bears an acorn, and the vine a gourd, man bears a poem . . . since his song is a vital function like breathing, and an integral result like weight” (A Week on the Concord and Merrimack Rivers, 1849).
The premise of all theories of organic form is that form should not be prescribed or fixed but should emerge from the subject matter at hand. It should, as Emerson said, “ask the fact for the form.” Ezra Pound formulated an imagist version when he wrote, “I think there is a ‘fluid’ as well as a ‘solid’ content, that some poems may have form as a tree has form, some as water poured into a vase” (1918). In the 1960s, Denise Levertov and Robert Duncan developed a more broadly theological concept of organic form. They believed that the form of the individual poem intuits the divine. Thus Levertov defined organic form as “a method of apperception, i.e., of recognizing what we perceive, and is based on an intuition of an order, a form beyond forms, in which forms partake, and of which man’s creative works are analogies, resemblances, natural allegories” (“Some Notes on Organic Form,” 1965). Duncan suggested that the poet “seeks to penetrate to that most real where there is no form that is not content, no content that is not form” (“Toward an Open Universe,” 1966).
In literary criticism and aesthetics, the word organic is commonly used to indicate the interrelationship between the parts of a work. We are employing a metaphor from nature when we say that things have an organic relation or organic connection, meaning that they seem to occur “naturally” rather than being imposed “artificially.”