I have placed this in the theory category because it concerns the use of allusion in all literature and uses as evidence examples from an ancient text from Rome and modern ones from England and the USA. I realize I have constructed a somewhat discursive argument. Perhaps some readers will find relish the casual rolling motion with which I turn from one topic to another. I have long felt that literary scholars, who of all people are aware that form is indeed content, are far too slavish followers of a single highly standardized pattern in their own essays.
Though many before me have noted the links from Horace to Dowson to Porter, I believe my use of them to make a point about allusion is original.
Horace is a great poet of self-contradiction. He can write convincingly on behalf of voluptuary pastimes as well as of frugal sobriety; he is at once a man of patriotic idealism and a slavish sycophant; a common-sense Everyman of conventional opinions one moment, he is a decadent aesthete the next. The opening ode in his fourth book which begins “Intermissa, Venus” seems a sincere and moving statement of middle-aged erotic ambivalence until one notes Suetonius’ comment that he returned to the form after having taken leave of it only at the command of Augustus. (Indeed, some critics view the entire fourth book as an artifice to contain the poems praising the reign of the emperor whom the poet once opposed on the battlefield.)
The persona begs Venus for relief, pleading age (he is fifty) and suggests she settle on a younger, more appropriate man to afflict with love, perhaps a certain Paulus Maximus. Should he find erotic success, this man will surely make her a worthy sacrifice – which the poet then describes in such extraordinary sensual terms – evoking visions of flutes, lyre, and pan-pipes and a chorus of dancing boys and girls -- that the poet finds himself aroused in spite of his intentions. As if to convince himself he declares he no longer takes pleasure in amours, “nec femina nec puer,” not even in drinking. Yet this very claim leads instantly to a tear at the thought of his frustrated love for Ligurinus which so affects him that he claims to be stricken silent, though the poem works its way without pause. The piece ends with a poignant image of the poet’s one-time lover receding in a dream, pursued across the Campus Martius, over streams, always just beyond reach, an eloquent image for desire.
This scenario is beautiful and moving in itself, far more of course in Horace’s ever-so-artfully chosen words than in paraphrase, but the modern reader is likely to miss a significant element in the author’s intention. Even many who can read the Latin are unskilled in meter. Classical verse forms are difficult for English speakers to appreciate. The very use of quantitative meters and the rich variety of available patterns, each of them flexible in prescribed ways, produce effects virtually impossible to reproduce in translation. The reading of a really accomplished classicist who can melodiously respect vowel quantity in Greek or Latin lyric poetry while also observing accent and pitch in a fluid and expressive flow of words is very rare and very beautiful, a phenomenon, like a string quartet well-played by a one-man band.
Still, the modern reader can scarcely doubt the importance in Horace’s own judgment of his use of particular meters. To support his claim to lasting fame the sole specific Horace offers (in 3.30) is that he, in the words of Pound’s translation, "first brought Aeolic song to Italian fashion" (“princeps Aeolium carmen ad Italos deduxisse modos”), in other words, he imported Greek metrical patterns adapting them for the Latin language.
The place of Greece as a source for Roman science, philosophy, and art was acknowledged, paralleling in some ways the role of China for Japan, though one might also cite the importation of Continental verse forms to England by Chaucer. The neoteroi of Catullus’ generation admired the Alexandrian Greeks, and Horace himself spent time in Athens doing advanced studies. So the initial meaning of a poet’s using Greek meters would be simply to identify himself as a savant. Horace’s use of the term Aeolian points directly to the work of Alkaios and Sappho, and there can be little doubt that the Latin poet intended for associations from the earlier writers to be part of his own effects. His use of Greek prosody constituted an allusion, implying that his own words could be fully understood only in connection with theirs. Commentators have long noted specific parallels with the two Lesbian poets, but in the broadest sense it is surely their passionate expression of love is their primary association.
Just as Doric was associated with choral odes in Greek and the Epic literary dialect with Homeric or mock-Homeric content, the use of Aeolic carried erotic connotations. Horace, writing in Latin, could not use these Greek dialects but he could use the earlier poets’ meters. Many of his odes are in Sapphics and Alkaics, while “Intermissa, Venus” is in Asklepiadics, another pattern rich in choriambs. While most of the poetry of Asklepiades is lost, enough remains to suggest its likely character. Two versions by the Imagist poet Edwin A. Storer will provide a sample of Asklepiades’ tone 
The Crown of Spring
Sweet for the thirsty in summer is snow to drink; sweet for sailors after winter’s storms to see the crown of spring, but sweeter still when beneath one cloak two lovers lie, giving their thanks to Kypris.
At the Porch
It is winter and the night is long. The Pleiades have travelled half their span, and I am passing by this door all wet with the rain.
Suffering from her treachery, I long for her.
O Kypris, it is not love you have sent me; it is some cruel shaft tipped with flame.
It is clear that the meter for Horace was a significant allusion, a sign of the sort of verse he meant to compose, a code that has in modern times become obscure except to specialists. This is, of course, unsurprising, considering that Latin has been little spoken for a millennium and a half and that English speakers are not generally conscious of vowel quantity, but the fact is that most modern readers of English poetry are nearly equally deaf to the associations of poetic forms in their own language. People accustomed to the most casual and colloquial free verse have for the most part lost the ability to catch allusions in prosody apart from the very simplest. Tetrameters are likely to be perceived as more natural than pentameters and the sonnet remains recognizable, but the associations of most meters and stanza forms are lost on the general reader, resulting in a significantly impoverished reception of texts both old and new.
The fin de siècle British writer Ernest Dowson uses a line from Horace’s ode as the title of his “Non sum qualis eram bonae sub regno Cynarae,” though his meter is not English, Greek, or Latin, but rather French Alexandrines. In Horace the line “I am not as I was under the rule of the good Cynara” is nostalgic, referring to the speaker’s advancing age which, he says, has rendered him a poor candidate for Venus’ attentions. For Dowson on the other hand Cynara is his current courtly beloved, unattainable and perfect, of whom he thinks while having sex with a prostitute.  Like most love songs, this is a lack-love song. To be sure, the speaker has the kisses of a “bought red mouth,” but he is “desolate,” “sick,” and haunted by desire even as he calls “for madder music and for stronger wine.”
The basis for the allusion, the similarity of the two Cynaras, is simply their unavailability. Whereas Horace had initially complained only of the universal decline that comes with age, Dowson possesses only too much vital energy, finding that no matter how “riotously” he flings roses, he cannot banish the shadow of the one he truly loves. The memory of Horace then simultaneously reinforces the fundamental feeling, the pain of the lover’s absence, and, by the contrast between the ancient and the modern poet, heightens for Dowson’s reader the lurid dramatic situation of Dowson’s persona. To the reader familiar with Horace, the allusion first engages by what is shared by the two unsatisfied lovers, but its effect is then complicated and enriched by their differences. Allusion quite commonly has an ironic intent, and the bathetic fall from ancient ideals to a reduced modernity is familiar from countless twentieth century works of which Joyce’s Ulysses is perhaps the most prominent example. Here, as in a great many instances, the allusive reference suggests an entire complex of relationships, some direct, some inverted, some otherwise skewed.
A far greater gap than that between the lyrics of Horace and Dowson is that between Dowson and Cole Porter who appropriated Dowson’s chorus line that ends each stanza “I have been faithful to thee, Cynara! in my fashion” for his song from Kiss me, Kate “Always True to You in My Fashion.” Here Lois seeks to reassure her lover Bill, uneasy about her liaisons with sugar daddies, a full dozen of whom are detailed. The list retains the listener’s interest as it ranges from the expected (“a big tycoon in steel,” “an oilman known as ‘Tex’”) to the unlikely (“a wealthy Hindu priest”) and the edgy (“a madman known as Mack”), and concluding with a real person, Clark Gable. The tone is entirely flippant, but the singer proclaims her loyalty to her Bill, asking
How in hell can you be jealous
When you know, baby, I'm your slave?
I'm just mad for you
And I'll always be
Porter’s playful amorality has nothing whatever in common with Dowson’s tortured and obsessive love. Indeed, for those familiar with the earlier poem, the allusion only enhances the libertine freedom of Porter’s high-spirited lyric. Further, one may safely assume that most listeners will be innocent of any knowledge of Dowson. In spite of this, his poem has left its traces by providing Porter’s starting point, and it underlies the thematic impulse of the lyrics.
The song’s other allusions, mentions of Schlitz beer, Cadillac cars, and a “vet” (the show was staged in 1948), as well as the reference to Clark Gable, serve to provide a patina of contemporary relevance, a kind of sparkling topicality, that invites the Broadway audience members to believe that the song is about their own lives. This is not so very different from Horace’s references to Venus and Cupid as well as Cynara, Ligurinus, and Paulus Maximus, the last three of whom are thought to be historical figures.
Allusion, like other rhetorical figures,  characterizes the aesthetic text. The study of rhetoric in this sense was for centuries the heart of literary theory and the foundation of practical criticism. In general such figures permit the expression of new thought and content that could not be formulated in ordinary transparent prose as well as aspiring to beauty. All literature, of course, must be built from the base of the previous body of writing. Pointed instances of allusion may have a number of functions, among them to vault the new text into the realm of art (in intention at least), to indicate universality, to decorate (either aesthetically or intellectually), to compress information, to thicken the meaning of all poetry, and ironic or other troping on the earlier text. This series from Horace to Dowson to Porter illustrates the importance and yet the subtle complexity and variety of allusive references, a figure of speech that requires the audience’s competent familiarity with the pre-existing tradition.
1. The Windflowers of Asklepiades and The Poems of Poseidippos, translated by Edward Storer; from The Poet’s Translation Series, Second Set, No. 5; London: The Egoist Press, 1920; pp. 3-17. The translations are also viewable on line at http://elfinspell.com/ClassicalTexts/Poetry/Asklepiades-Poseidippos/Storer-Asklepiades.html.
2. Biographical critics will identify her with Adelaide "Missie" Foltinowicz, the restauranteur’s daughter and sometime waitress with whom he had been smitten when she was eleven. When she was fifteen, he proposed to her and was rejected.
3. I will not trouble myself over the terminology of rhetorical figures which have been classified also as tropes and schemata and analyzed into figures of thought, of speech, of sound, and of syntax, For the present purpose a rhetorical figure is any usage by which a written passage conveys meaning beyond the direct literal signification.