Alfred Russel Wallace discovered natural selection independently of Charles Darwin. Through his letters, available online for the first time, he tells us of his research, expeditions and enduring fascination for nature's mysteries.

You are famously joint author, with Darwin, of the first paper describing the origin of species and natural selection, published in 1858. When did you first get the idea?
I begin [in 1847] to feel rather dissatisfied with a mere local collection – little is to be learnt by it. I sh[ould]d like to take some one family, to study thoroughly – principally with a view to the theory of the origin of species. By that means I am strongly of [the] opinion that some definite results might be arrived at.

This desire led you to Brazil to collect birds, butterflies and beetles to try to discover what drives the evolution of new species. Were there any incidents on the voyage?
On Friday the 6th of August [1852]… the Captain (who was the owner of the vessel) came into the cabin & said "I am afraid the ship's on fire. Come & see what you think of it."

Despite that harrowing experience, you next undertook an 8 year expedition to the Malay Archipelago, where you discovered the invisible boundary between the animals of Asia and the Australian region, which would later be called the Wallace Line in your honour. What fascinated you most on that trip?
The Birds have however interested me much more than the insects, they are proportionally much more numerous, and throw great light on the laws of Geographical distribution of Animals in the East… As an instance I may mention the Cockatoos, a group of birds confined to Australia & the Moluccas, but quite unknown in Java Borneo Sumatra & Malacca… Many other species illustrate the same fact.

You have been famously good-natured about sharing the discovery of natural selection with Darwin…
I also look upon it as a most fortunate circumstance that I had a short time ago commenced a correspondence with Mr. Darwin on the subject of "Varieties", since it has led to the earlier publication of a portion of his researches & has secured to him a claim to priority which an independent publication either by myself or some other party might have injuriously effected

What did you and Darwin have in common?
In early life both Darwin and myself became ardent beetle-hunters. Both Darwin and myself had, what he terms "the mere passion of collecting"… Now it is this superficial and almost child-like interest in the outward forms of living things, which, though often despised as unscientific, happened to be the only one which would lead us towards a solution of the problem of species.

Do you feel your contribution has been overlooked?
The idea came to me, as it had come to Darwin, in a sudden flash of insight: it was thought out in a few hours – was written down with such a sketch of its various applications and developments… then copied on thin letter-paper and sent off to Darwin – all within one week.

I should have had no cause for complaint if the respective shares of Darwin and myself in regard to the elucidation of nature's method of organic development had been thenceforth estimated as being, roughly, proportional to the time we had each bestowed upon it when it was thus first given to the world – that is to say, as 20 years is to one week.

You helped Darwin with the puzzle of bright colouration in animals, which led to the concept of warning colours. To ask his question again, why are some caterpillars so brightly coloured?
[Since some]… are protected by a disagreeable taste or odour, it would be a positive advantage to them never to be mistaken for any of the palatable caterpillars… Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable caterpillars, would enable birds to recognise them easily as a kind not fit for food, & thus they would escape seizure which is as bad as being eaten.

How did you feel looking back on your life's work, at the age of 89?
The wonders of nature have been the delight and solace of… life. Nature has afforded… an ever increasing rapture, and the attempt to solve some of her myriad problems an ever-growing sense of mystery and awe.

Do you have a message for our readers?
I sincerely wish you all some of the delight in the mere contemplation of nature's mysteries and beauties which I have enjoyed.

HOW do you spot a fit partridge? Check out the fractals round its neck.

Fractal geometry is used when a pattern is too complex to be described by Euclidean geometry. It has been applied to coastlines, plant structures and animals' foraging patterns. Lorenzo Pérez-Rodríguez at the National Museum of Natural Sciences in Madrid, Spain, and colleagues wanted to see if it could also be helpful when analysing the complex plumage patterns of birds.

Using images fed into software, the team found that red-legged partridges with a more gradual transition between the plain and spotted areas of their bib have a higher fractal dimension (FD) - a measure of a pattern's complexity.

To see if this was linked to the bird's fitness, they compared the bibs of 68 birds of both sexes, half of which were on a restricted diet. After six months, the bibs of undernourished birds had a lower FD than before their food was reduced. Low FD also predicted poorer immune responsiveness.

Pérez-Rodríguez thinks that a fractal-rich bib could be used to advertise the health of the bird to potential mates. "Birds have quite a different visual system to ours," says Thanh-Lan Gluckman at the University of Cambridge, so the work could also help us understand what one bird sees in another.