The story of how Alfred Russel Wallace startled Charles Darwin into writing On the Origin of Species is well known. In June 1858, Darwin received a bombshell letter from Wallace, who at the time was on the far-flung Malay Archipelago. After many years’ research, Darwin had spent the last four years working on his long-planned ‘big book’ on species, having twenty years earlier arrived at the idea of natural selection as a mechanism for evolutionary change. In his letter, Wallace described essentially the same mechanism. Poor Darwin was devastated, but felt honour-bound to publish Wallace’s paper. Anxious that he should receive due credit for having come up with the idea first, and for having worked on it for two decades, Darwin’s close friends and confidants Charles Lyell and Joseph Dalton Hooker arranged for the hasty publication of Wallace’s paper alongside two unpublished documents written by Darwin. To mitigate the risk of being scooped any further, Darwin immediately set aside his ‘big book’ (which he never got round to finishing) and began writing an ‘abstract’ of it that was to become On the Origin of Species.
Modest almost to a fault, Wallace always expressed complete satisfaction at how his paper was published, and insisted it was only right that Darwin’s name became synonymous with the idea of natural selection:
As to the theory of “Natural Selection” itself, I shall always maintain it to be actually yours & your’s only. You had worked it out in details I had never thought of, years before I had a ray of light on the subject, & my paper would never have convinced anybody or been noticed as more than an ingenious speculation, whereas your book has revolutionized the study of Natural History, & carried away captive the best men of the present Age. All the merit I claim is the having been the means of inducing you to write & publish at once.—Alfred Russel Wallace to Charles Darwin, 29 May 
Darwin and Wallace were to remain friends for the remainder of Darwin’s life. Wallace dedicated his magnificent book The Malay Archipelago to Darwin, ‘not only as a token of personal esteem and friendship but also to express my deep admiration for his genius and his works’. Late in his own life, Darwin was instrumental in securing a Civil List pension for the ever-cash-strapped Wallace.
Darwin seem to have seen Wallace as someone to bounce ideas off—not least because, unsurprisingly, Wallace had a deep understanding of natural selection. One wonderful example of this was when Darwin became utterly flummoxed as to why many caterpillars sport gaudy colours. Bright colours would make them easier for predators to spot. Darwin’s usual explanation for apparently disadvantageous colouration was sexual selection: bright colours being more attractive to potential mates. But caterpillars don’t mate until they have metamorphosed into butterflies, so their bright colours could not be explained in this way. As Darwin later recalled in The Descent of Man, ‘I then applied to Mr. Wallace, who has an innate genius for solving difficulties’. Wallace responded the following day:
Now great numbers are protected by their green colours assimilating with foliage or their brown colours resembling bark or twigs. Others are protected by prickles and long hairs—which no doubt render them distasteful to birds, especially to our small birds which I presume are the great destroyers of catterpillars. Now supposing that others, not hairy, 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 catterpillars, because a slight wound such as would be caused by a peck of a bird’s bill almost always I believe kills a growing catterpillar. Any gaudy & conspicuous colour therefore, that would plainly distinguish them from the brown & green eatable catterpillars, 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.—Alfred Russel Wallace to Charles Darwin, 24 February 
Wallace had identified a new biological concept to explain Darwin’s conundrum: warning colouration. In the same letter, he went on to describe how this hypothesis might be tested experimentally. The amateur entomologist John Jenner Weir performed such experiments a few months later.
Not all of Wallace’s advice to Darwin was quite so useful. On 2nd July 1866, he wrote to his friend urging him to drop the term natural selection in favour of Herbert Spencer’s alternative term, survival of the fittest. Wallace felt many people incorrectly assumed the term natural selection implied ‘the constant watching of an intelligent “chooser”’. So strongly did Wallace feel on this topic that he went through his own copy of On the Origin of Species, systematically crossing out the phrase natural selection and replacing it with survival of the fittest. Darwin followed Wallace’s advice and adopted the phrase survival of the fittest, first in his book The Variation of Animals and Plants under Domestication, then in later editions of On the Origin of Species. Nowadays, Wallace’s preferred phrase is seen as potentially misleading, as many people incorrectly assume the word fittest means physically fittest, rather than best fitted (i.e. best adapted to an environment and lifestyle).
Darwin and Wallace by no means agreed on everything, but, when they did disagree, it was always amicably. Darwin, for example, placed more emphasis on competition between individuals of the same species as a driver for natural selection, whereas Wallace placed more emphasis on environmental drivers. Wallace was unimpressed with Darwin’s ideas on instinct, and pretty much rejected his idea of female preference as a driver for sexual selection. Rather than certain male birds evolving bright colours to attract females, Wallace suggested the female birds might have evolved drab colours to better camouflage them while they were sitting on their nests. After initial enthusiasm, Wallace also eventually rejected Darwin’s (very wrong) hypothetical mechanism for heredity, pangenesis.
One important area in which Darwin and Wallace were initially in strong agreement, but later differed, was on the application of the theory of evolution by means of natural selection to the human mind. Both initially agreed human intellect and morality must have evolved. But, in 1869, Wallace, who had developed an interest in spiritualism, went through a major about-face, admitting to his friend:
In my forthcoming article in the “Quarterly”, I venture for the first time on some limitations to the power of natural selection. I am afraid that Huxley & perhaps yourself will think them weak & unphilosophical. I merely wish you to know that they are in no way put in to please the Quarterly readers,—you will hardly suspect me of that,—but are the expression of a deep conviction founded on evidence which I have not alluded to in the article but which is to me absolutely unassailable.—Alfred Russel Wallace to Charles Darwin, 24 March 1869
Ironically, the man who had argued against Darwin’s use of the term natural selection because it could be seen as implying an ‘an intelligent “chooser”’ now believed some form of greater power must have been at work in the development of the human mind. Darwin’s initial response, before reading Wallace’s article, displayed typical melodramatic humour: ‘I hope you have not murdered too completely your own & my child’, but his subsequent response on reading Wallace’s article displayed genuine concern:
Altogether I look at yr article as appearing in the Q-ly as an immense triumph for our cause. I presume that yr remarks on Man are those to which you alluded in yr note.
If you had not told me I shd have thought that they had been added by some one else. As you expected I differ grievously from you, & I am very sorry for it. I can see no necessity for calling in an additional & proximate cause in regard to Man.—Charles Darwin to Alfred Russel Wallace, 14 April 1869
But even important differences like this need not damage a friendship. The following year, Wallace was to write in the preface to a book of his essays:
I have felt all my life, and I still feel, the most sincere satisfaction that Mr. Darwin had been at work long before me, and that it was not left for me to attempt to write “The Origin of Species”.—Alfred Russel Wallace, Contributions to the Theory of Natural Selection (1870)
Darwin’s response summed up the two men’s friendship wonderfully well:
There never has been passed on me, or indeed on any one, a higher eulogium than yours. I wish that I fully deserved it. Your modesty and candour are very far from new to me. I hope it is a satisfaction to you to reflect,—& very few things in my life have been more satisfactory to me—that we have never felt any jealousy towards each other, though in one sense rivals. I believe that I can say this of myself with truth, & I am absolutely sure that it is true of you.—Charles Darwin to Alfred Russel Wallace, 20 April