Charles Darwin has been lauded as the principal author of evolutionary theory. But should the laurels be awarded instead to his grandfather, Erasmus? Desmond King-Hele examines the evidence

It has often been said that Erasmus Darwin, grandfather of the more famous Charles, achieved more in a wider range of intellectual disciplines than anyone since. In a biography to be published next year I suggest that not only did Erasmus adopt the idea of biological evolution around 1770, long before his grandson's birth, he also grasped the mechanism of evolution - which we now call natural or sexual selection.

Erasmus Darwin was by profession a physician - regarded by many as the leading doctor of the day. He was also a poet. After the publication in 1792 of his long poem "The Botanic Garden", which presented science to the public in a form they could understand, Erasmus was recognised as the leading English poet of the time - a fame which lasted for about five years.

But Erasmus's most impressive talent was his scientific insight, a gift for seeing how nature functions, right across the spectrum of science. In physical science he was the first to state the fundamental gas law of adiabatic expansion, which recognises that air cools when allowed to expand from a state of high pressure to lower.

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In meteorology he recognised the importance of cold and warm fronts and suggested weather maps. In biological science he was the first to specify the complete process of photosynthesis; he argued that carbon, nitrogen, phosphorus and calcium were essential to plant nutrition; and offered other ideas such as biological control of pests. These insights were years ahead of their time, as was his belief in biological evolution.

Erasmus adopted the idea of biological evolution (as we now call it) about 1770, when he was 38. He had helped Wedgwood with the promotion of the Trent-and-Mersey canal, and in 1767 many large bones were dug out at the cutting of the canal tunnel at Harcastle, near Stoke. As a doctor Erasmus was asked to identify them. He could not. Realising that they were relics of unknown species he soon came to the conclusion that species had changed.

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He was a convinced Enlightenment man, strongly imbued with the idea of progress. That led him to the idea of the progress of species from the simple to the complex. By 1770, he had arrived at the belief that all species were descended from a common microscopic ancestor - the theory of common descent as we now call it.

How could he publicise the revelation without being ridiculed? Only by stealth. He decided to add the motto E conchis omnia, "everything from shells", to his family coat of arms, which were (conveniently) three scallop shells. He had the arms and motto painted on his carriage and thought no one would realise the implications. Unfortunately Canon Seward of Lichfield Cathedral did. Seward attacked Erasmus in satirical verses, accusing him of "renouncing his Creator": Great Wizard he! By magic spells Can all things raise from cockle shells...

Erasmus could not insult the church, because most of his income came from rich patients who were pillars of the church. So he had to paint out the motto on his carriage, though he kept it on his bookplate. After this humiliating debacle, he remained silent about evolution for 20 years.

At the age of 60, when he was famous as a poet, Darwin decided to publish his big book about medicine and animal life, Zoonomia. Being "too old and hardened to fear a little abuse", he risked putting in a chapter about evolution. But by the time the book was published in 1794 England and France were at war. The government suspended habeas corpus; respectable reformers were being sent to Botany Bay for 14 years; and the trials of Horne Tooke and others for treason were about to begin. The need was for books that supported church and state, not subversive theories that undermined religion by denying God his role as creator of species and sapped national morale by implying that Britons were descended from apes.

In fact Darwin tactfully avoids mentioning apes in Zoonomia, but the implication is there. He argues that changes in species are possible because many creatures change in form, such as tadpoles turning to frogs. Also monstrosities (or mutations as we might say) are often inherited, he says. Such evidence of change, and the similar structure of all warm-blooded animals, he says, led him to believe that all animals may have "a similar living filament" as a common microscopic ancestor.

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He was well aware of what we call natural selection and sexual selection. In species where males fight each other for possession of the females, the outcome is, he says, "that the strongest and most active animal should propagate the species, which should thence become improved". The time-scale from the microscopic filament to the present, he suggests, would be "millions of ages", that is, hundreds of millions of years.

Darwin was criticised, ridiculed and satirised for his evolutionary views in the late 1790s. Because of the hostility he lost his pre-eminent status as a poet. But in his final years he wrote a second poem, "The Temple of Nature" or "The Origin of Society", where he traces the evolution of life from microscopic specks in primeval seas through fishes and amphibians to humankind, as he often calls us. He summarises evolutionary development in three couplets: First forms minute, unseen by spheric glass, Move on the mud, or pierce the watery mass; These, as successive generations bloom, New powers acquire, and larger limbs assume; Whence countless groups of vegetation spring; And breathing realms of fin, and feet, and wing.

Later in the poem he looks at the web of slaughter in nature - a vivid picture of the struggle for existence in the realms of "air, earth and ocean". He goes on to show how humans have sometimes succeeded in helping themselves by creating society.

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The poem was savagely criticised. The reviewer in the British Critic could not take it and gave up: "We are full of horror and will write no more." People were not ready for evolution and Erasmus failed to convince them of its truth.

Erasmus's son Robert followed his father's views on abstaining from alcohol, treating religion with some scepticism and believing in evolution. But Robert was shocked by the controversy which had greeted his father's suggestions: he kept very quiet about evolution. Did he, nevertheless, believe in it?

Robert Darwin knew the trouble caused in 1770 by E conchis omnia; yet he still put it on his own bookplate. So he probably deserves credit for having brought up his own son Charles in an evolution-friendly family atmosphere, which helped him escape early from a belief in the conventional special-creation theory.

Charles had read Zoonomia at the age of 18. "At this time," he says, "I greatly admired the Zoonomia." He read the chapter on evolution, "but without producing any effect on me". This was true at the conscious level, otherwise he would have been an evolutionist ten years earlier. But Charles probably felt uneasy about having been less than generous to his grandfather. He made up for it by his extraordinary decision, at the age of 70, to write a biography of Erasmus, in which he made many kind remarks about his grandfather - nearly all of which were censored by his family before publication. And Erasmus, had he still been alive, would surely have made even kinder remarks about the achievements of his grandson. Evolution was the family faith.

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Desmond King-Hele is a fellow of the Royal Society and author of the forthcoming Erasmus Darwin: A Biography, to be published in 1998.