The first drafts of Charles Darwin's theory of evolution, which have never before been seen in public, are being published online for the first time today.

For decades one of the most important collections of primary materials in the history of science was available only to scholars at Cambridge University Library, which had been given the 90,000 papers and images by the Darwin family in 1942.

They were delivered in parcels containing small packets of manuscript wrapped in tissue paper on which the subjects had been noted in Darwin's hand, just as he had left them.

The papers, now published by Darwin Online, include early calculations on his theory of evolution, thousands of drafts of his scientific writings, records of his experiments, and even arguments in favour of a wife, who he described as "better than a dog anyhow".

Darwin was just 22 when he joined HMS Beagle as resident naturalist for a five-year world voyage. When he arrived in the Galapagos Islands, the inspiration for On the Origin of Species, he was astonished by the creatures he encountered, such as the centenarian giant tortoises, from which the islands took their name.

On his return to Britain in 1836, armed with thousands of geological specimens from his trip across South America, he began devising a new theory of evolution.