In the first few months of 1858, Darwin had a letter from Wallace, who had for some years been exploring the archipelago now known as Indonesia, in which he told him about the grand theory he had developed about how small changes might give animals an edge over competitors and therefore mean greater success for their species.

Wallace had hit on the idea of natural selection - the same idea Darwin had been quietly working on for decades. Darwin urged him to publish and it was only at the insistence of his close friends Charles Lyell and Joseph Dalton Hooker that Darwin included his own work in the letter that would be read to the society on July 1, 1858.

Neither man was present at the meeting. Darwin was represented by his friends and Wallace was still in Southeast Asia.

Both men had been struck by the fact that if species were able to reproduce unchecked, then the world would very quickly become overrun. They pondered what sort of checks nature placed on certain types of animals that limited their proliferation.

They both realised nature's resources were finite and that even though many animals might fall victim to predators, far greater numbers were dying because they simply could not compete for limited resources.

Darwin's contribution to the Linnean reading was an excerpt of a manuscript he had written in 1844 and a letter to a close friend in which he further expanded on his idea.

He gave the example of a dog that lived mainly on rabbits and occasionally ate hares, imagining what would happen if rabbits for some reason suddenly became scarce.

'Those individuals with the lightest forms, longest limbs, and best eyesight - let the difference be ever so small - would be slightly favoured [in terms of chasing the faster hares], and would tend to live longer, and to survive during that time of the year when food was scarcest; they would also rear more young, which would tend to inherit these slight peculiarities.

'The less fleet ones would be rigidly destroyed. I can see no more reason to doubt that these causes in a thousand generations would produce a marked effect, and adapt the form of the fox or dog to the catching of hares instead of rabbits, than that greyhounds can be improved by selection and careful breeding,' Darwin wrote.

Wallace had come to the same conclusion. 'Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals,' he wrote in his letter to Darwin.

'Even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they inhabit. It is also evident that most changes would affect ... the powers of prolonging existence.

'In the wild animal ... all its faculties and powers being brought into full action for the necessities of existence, any increase becomes immediately available, is strengthened by exercise, and must even slightly modify the food, the habits, and the whole economy of the race. It creates, as it were, a new animal, one of superior powers, and which will necessarily increase in numbers and outlive those inferior to it,' Wallace wrote.

But while Wallace had hit upon the same idea as Darwin, history has tended to remember Darwin as the father of evolution - and it was his publication the next year of On the Origin of the Species that put the concept before the eyes of the public.

That has caused some to suggest that Darwin stole Wallace's idea and made it his own, a view rejected by British biologist and author Richard Dawkins, arguably one of the foremost proponents of the theory.

During a discussion at the Cheltenham Science Festival early last month, Professor Dawkins said Darwin had been chipping away at his theory for a number of decades before receiving Wallace's letter, and that his ideas about natural selection had been forming since he took a job as a 'gentleman companion' for the captain of a survey ship named the Beagle.

'He noticed all sorts of things that were beginning to ferment in his mind,' he said of the slight differences Darwin noted between animals on different land masses.

'This started to bubble away in Darwin's mind. [But] he first got it really right in his mind in the early 1840s, and then waited almost 20 years before he went public.

'I find it astonishing that he wasn't afraid of being scooped, because it's such a simple idea, and such an immensely powerful one, that you'd think he'd be worried about someone else coming up with it - as indeed they did in 1858.

'There are people who like to champion Wallace in that respect. It is true that when Wallace sent [the letter] it prompted Darwin into action. So there is a certain amount of justice in saying that while Darwin didn't steal Wallace's idea, that Wallace, having had the idea independently, stimulated Darwin into action.'

And hence the reading of the theory at the Linnean Society meeting. 'This was a nice, gentlemanly way of sharing the credit. Unfortunately, in 1858 ... not many people took any notice [of the announcement],' Professor Dawkins said.

It would be some time before the significance of the event was appreciated, but at the same time the way in which the men conducted themselves has also been a talking point through the years.

'Both men were entirely original, and all the world knows how generously each discoverer strove to secure full recognition for the claims of the other,' The Times wrote in 1908, on the 50th anniversary of

the meeting.

'It would be better for science and for the world if the example they set in that respect were as much regarded as the theory they jointly promulgated. 'Their treatment of one another's claims is a standing rebuke to the mean jealousies that too often disfigure scientific intercourse at the present day, and to the eager desire to secure priority, which frequently leads to the publication of crude conceptions and dubious results.'

The 1908 editorial went on to note that in the 50 years since its promulgation, great progress had been made towards proving the theory, but also that it would be the subject of argument, both scientific and otherwise, for a good time to come. The paper wasn't wrong. Even 150 years later there are those who doubt the theory's veracity.

Author Josh Greenberger is an advocate of intelligent design - a theory that holds that various forms of nature are too complex to have come about through the random and brutal process of selection advocated by Darwin and Wallace.

'What has kept evolution alive for so many years,' said Greenberger, 'is its attractiveness rather than its scientific validity. Darwin's evolution offered even the intellectual a way out of believing in God; if life can create itself and spawn millions of species by accident, what need is there for God?' Greenberger claims that if evolution holds true, and small changes have massive results and result in huge numbers of casualties, there should be far more evidence of unsuccessful species.

'It means, even before natural selection comes into play, random genetic mutations would have to produce millions of deformed species for every viable one. The fossil records show no signs of profuse quantities of deformed species - which leads to only one conclusion: the evolution of life could not possibly have been an accident.'

Ecologist Cheung Siu-gin, an associate professor at City University's department of biology and chemistry, disagrees.

'There are many experiments that are working on this subject with bacteria and micro-organisms and they are showing that evolution is happening,' he said.

The theory of evolution had given scientists an incredibly powerful tool with which to improve their understanding of the world.

'Nowadays, most ecological theories are based on the development of evolutionary theory,' Professor Cheung said. 'Evolution can go in different directions, depending on the selective forces at play.

'People ask about how come an animal behaves in a certain way - is that a product of evolutionary forces?'

Professor Cheung cited the example of humans' taste for things alcoholic. 'People think there may be an evolutionary influence in alcoholism. Researchers have shown that there are certain primates that actively search for fermenting fruit.

'We are also primates, so we can be expected to have some similar behaviour. We developed a liking for alcohol because it helped [our ancestors] find ripe fruit - something with a lot of energy.'

Life, for pretty much every organism on the planet, the professor said, was essentially a race for energy to live and to grow strong enough to breed.

'Say, we are looking at different groups of primates. Every individual in a species is different and, because of their genetic makeup, some will be better at smelling out the fermenting [and therefore ripe] fruit.

'Over time, those primates will be better at finding energy and will be more successful and therefore breed more and pass on their genetic characteristics.'

And that, according to Professor Cheung, is why we like alcohol. So, a toast ... to evolution!