For Jeremy Bentham, a philosopher, poetry was simply writing that “fails” to reach the end of the line. For W.H. Auden, a poet, poetry was that which “makes nothing happen”. Arnold Bennett, a writer, disagreed: he thought poetry was very powerful. The mere word “poetry” could, he said, “scatter a crowd” faster than a firehose.

What unites these descriptions of poetry is that none uses the word “rhyme”. When A.E. Housman, a poet, gave a 51-page lecture titled “The Name and Nature of Poetry” in 1933 he used the word “rhyme” just once, and then only in the phrase “bad rhyme”. Martin Heidegger, in an essay titled “What Are Poets For?” (1946) was similarly avoidant: the philosopher used the word “abyss” 16 times, “death” five - and rhyme not once.

Poets rarely define poetry by whether or not it rhymes. This is just as well, for now it hardly does. P.G. Wodehouse, a novelist, divided poetry into the “old-fashioned kind with rhymes in it” and the modern stuff about “gas-works and decaying corpses”. The Economist’s analysis of 11,000 poems in English finds verse firmly in the gas-works and corpses camp. In 1900, 80% of poems contained rhyme; today, only around 25% do. Numbers of rhymes fell too: at the start of the 20th century over 60% of lines rhymed; now under 5% do (see chart). Rhyme, says Wendy Cope, a poet, has become “pretty unfashionable”.

Poetry’s most prestigious prizes bear that out. The contenders for this year’s Griffin Poetry Prize - which is announced on June 4th and comes with a cheque for C$130,000 ($94,000) - have produced poems with titles such as “Unregulated Waste Management Facility”. T.S. Eliot prizewinners offer titles including “the body as cemetery”. All are rich in words like “penumbra” and in unexpected italics. They are not rich in capital letters at the start of sentences. or jokes. or rhymes.

This is historically unusual. Poetry used to have rhyme and rhythm: the reason both words are hard to spell is that they derive from ancient Greek. The oldest recorded rhyme is from China in the 10th century BC. This ubiquity and longevity hint that the brain is “hardwired” to notice rhymes, says Samuel Jay Keyser, author of “Play it Again, Sam: Repetition in the Arts”, a new book. Other things reveal this too, such as the sheer ease with which you can detect rhymes in a text. A rhythm can be hard for you to see. That last sentence, for example, was in iambic pentameter, but you probably did not notice. Noticing rhyme in text is easier: if it’s got it, you can spot it.

You used to be able to spot it a lot. There were children’s rhymes and adult rhymes, larky rhymes and snarky rhymes that asked bombs to fall on Slough as “It isn’t fit for humans now.” There were sorrowful rhymes that told you not to “Go gentle into that good night”, but “Rage, rage against the dying of the light”.

Then rhyme collapsed. John Milton - a poet who influentially decided to forgo rhymes in his epic poem, “Paradise Lost” - inflicted an early blow. But it was the modernists who killed off rhyme.

At the start of the 19th century, around half the population was literate; by the start of the 20th, 97% was. This was good for egalitarianism but bad for intellectual egos, since poetry does not merely confer pleasure but status. In an era of low literacy the mere ability to read a poem set someone apart; as the era of mass literacy dawned, another marker of intellect was needed. It came in the form of modernism.

In the 20th century, many artforms became “more abstruse, inaccessible and difficult to appreciate”, says Steven Pinker, a professor at Harvard University, “possibly as a way of differentiating elites from the hoi polloi”. Any fool can enjoy an enjoyable thing, but only a committed intellectual can enjoy an unenjoyable one. By the mid-century, rhyming lines had fallen by half.

Modernist verse is thus the peacock’s tail of poetry: something that evolved to be clearly hard to bear, but impressive if you can. Consider the epigraph of T.S. Eliot’s modernist masterpiece “The Waste Land”. It begins, forbiddingly, in Latin, then ends in ancient Greek with the words apothanein thelo (“I want to die”). Eliot can make everyone feel a bit like that.

Another possible cause of the decline is market forces - or their absence. Once, poets made money by selling poems: Lord Byron’s “The Corsair” shifted 10,000 copies in a day. Readers wanted rhymes, so poets provided them. But in the 20th century it became possible, as the poet Philip Larkin pointed out, to make a living less by poetry than “by being a poet”.

From bad to verse Look at a list of recent winners of any of the big poetry prizes and most will share three characteristics: you will not have heard of them; their poems will not rhyme; and they will have worked as poets in universities, peddling poetry as (partially) state-subsidised muses. This is poetry less as a paid-for product than as a literary utility: something that - like road surfacing or sewage disposal - is widely considered necessary for a civilised society but that no one wants to fork out for.

The poetry that does sell is produced by a new generation of social-media poets such as Donna Ashworth and Rupi Kaur. This is to the distress of intellectuals, for Instapoets’ verse is not the gas-works and cemetery kind. It is designed to be shared online, meaning that it is anodyne and often accompanied by line drawings of birds.

Ms Ashworth prefers to write about nice things. She celebrates your “inner voice”, your “inner light” and your “inner child” - which may make many readers feel in touch with their inner breakfast. She warns readers not to listen to their “inner critic”. (Given some reviews, Ms Ashworth may prefer not to listen to outer ones either.) She reveres hope: put hope, she advises, “beside your car keys” lest you lose it.

Naturally such poems do not rhyme. For rhyme, alas, seems to have been almost entirely lost. Perhaps no one remembered to put it beside the car keys. ■