I know of no paths to modern poetry that do not run through Baudelaire and the French Symbolists. There are easy and obvious reasons to do so. The Symbolists were formal innovators, more so than their Brit-late Victorian-Pre-Raphaelite contemporaries (think Tennyson, Swinburne, Browning to some extent, tho NOT Hopkins, who was doing something completely different). The Symbolists brought, more than anyone before them, the urban, the grotesque, and the physical/violent/erotic into their poetry. As a complement to this, they tended toward simpler, less "poetic" language (Baudelaire in particular), even as they produced works of a certain opacity.
All of these things would later be picked up, and perhaps perfected, in the poetics of modernism. But this isn't, I don't think, the most interesting line of connection from Symbolism to modernism. Or, better, these obvious similarities and influences are simply symptoms of a bigger aesthetic and philosophical shift, with numerous consequences, that the Symbolists inaugerated. I'd call that shift the attention to the word and its work, and I'll simplify a sketch of it here.
On a metaphysical level, the Symbolists were trying to get at the heart of things, the really real, which they tended to characterize, in a vaguely spiritual way, as the infinite, the unknowable, the absolute. Since it wasn't knowable or representable in any direct sense, one had to get at it by other means. Language, in its suggestive and affective qualities, was one of those means. The poetic symbol, not an iconic figure (as in Medieval poetry), was an emotionally charged thing that pointed to something bigger, something beyond itself, that could not be named or imitated through a direct mimesis. Thus, poetic lines like the following from Baudelaire's "Evening Harmony: "The sky like a lofty altar is sad and lovely; / The sun has drowned in its blood that is clotting yet." On the one hand, these lines appear to be simple metaphors for a certain quality of the evening sky, as it is emotionally colored "sad and lovely." But, when we look closer, the strange juxtaposition of emotional states and concrete details produces a sky we would never actually see in nature, a fuzzier, more fully imaginative landscape. The sky as a "lofty altar" upon which the sun has "drowned in its blood" possesses overtones perhaps of the sacrificial altar, upon which animals are killed, but, in this case, it is the sun that is drowning in its own blood, a sort of (strange) self-killing. The sun has slain itself in this grotesque, yet "sad and lovely" scene.
It is at once a highly emotional, pathetic poetic phrase, suggestive in its color and narrative, but simultaneously an unnatural one, not realistic. Moreover, the emotions it evokes cannot be simply described; they don't correspond to anything specific in everyday human emotional life. In other words, the poem creates an indefinite, even new, emotional experience out of the specific and imaginative use of language, channeling elements from the natural world but not imitating them. We cannot see the sky drowning in its own blood in nature, nor can we recreate the experience which would lead to this emotional response. It can only be found in the poem itself, and that indirectly, suggestively, symbolically.
If you cut off the metaphysical absolute oneness of Symbolist metaphysics (which isn't really necessary for understanding what they are doing poetically), you see a movement toward "pure art," or art cut off from society, history, ideas. This is not an art that is indifferent to the reader, however, a cold elevated structure of transcendence that leaves one feeling small. It is an art which actually breaks into the reader emotionally and affectively as art. At the same time, this pure art is absolutely constructued. It is the use of language as the material of art which makes symbolist poetry work. Thus, the word and words are cut free from stock associations or tales of Ladies of Shalott--they become material for new aesthetic constructions. The freedom and materiality of the word reaches its Symbolist high point in Mallarme's famous "A Throw of Dice Not Ever Will Abolish Chance" (Here's an html translation), probably, I would argue, the most important poem for what was to come after.
Jump forward a few years to the time of Pound, Eliot, Wyndham Lewis, Williams. The affirmation of the unknowable has deteriorated, partly for aesthetic reasons (the Vorticist distaste for British soupiness and Impressionist fuzziness would have, it seems, little tolerence for the Symbolist "infinite"). Indeed, Hugh Kenner reports that Pound had little tolerence for the vagueness of post-Symbolist poetry in France and otherwise--it had lost its aesthetic purpose. At the same time, a figure like Pound was interested in the conceptual power of the Image, "the material of poetry," as an emotional and intellectual structure. The lines from Baudelaire above have a certain proto-imagist quality to them, even if they lack the definition and precision of "Petals on a wet, black bough." In other words, the Symbolist interest in language and construction was maintained, albeit with harder lines. Vorticism was symbolism on steroids.
This is perhaps easier to explain in visual art. Wyndham Lewis, for instance, in his absolute commitment to art's self-sufficiency (not quite the same of l'art pour l'art), was dedicated to the force of lines and shapes. Consider this painting, "Timon of Athens:"

While not as representative as others, it is striking how much movement Lewis produces out of mostly straight lines. These are not impressions, they are objects, simple, sharp, abstracted objects put into definite constructions to produce a concrete effect. One can call it an emotional or imaginative effect, but it is far more abstract than "The sun has drowned in its blood that is clotting yet." Nevertheless, in creating something non-imitative from simple units, and, in doing so, unleashing a tremendous "objective" energy in and between those units, Lewis is stepping forward, I would argue, in the pathway the Symbolist first attempted to break. Force is not a Deleuze-Guattarian intensity here: it is a vortex, particles put in relation with one another that create an energy which cannot be found in their parts.
Of course, as a painting, the relation to thinking is vexed, if present at all. Thought has always been a problem for poets and philosophers, and part of Pound's taking leave of his London years in "Hugh Selwyn Mauberley" is a return to thinking. Nevertheless, the Cantos can be seen as an effort to take the vortex into historical, material thinking. A difficult task; the same attempted by "No ideas but in things."
Well put, sir, especially the words on Lewis. In a conference paper, I attempted to articulate Lewis's forces as "thinging movement," which I argued was the antithesis of Deleuze's image-movement in the Cinema books. I'm not sure I ever convinced myself, let alone the others in attendance.
Posted by: Jentery at March 26, 2007 11:12 PM