Science in the Soul Page 12

  G.S.

  * * *

  * In the foreword to David P. Hughes, Jacques Brodeur and Frédéric Thomas, Host Manipulation by Parasites.

  ‘More Darwinian than Darwin’: the Darwin–Wallace papers*1

  IT IS IN the nature of scientific truths that they are waiting to be discovered, by whoever has the ability to do so. If two different people independently discover something in science, it will be the same truth. Unlike works of art, scientific truths do not change their nature in response to the individual human beings who discover them. This is both a glory, and a limitation, of science. If Shakespeare had never lived, nobody else would have written Macbeth. If Darwin had never lived, somebody else would have discovered natural selection. In fact, somebody did – Alfred Russel Wallace. And that is why we are here today.

  On 1 July 1858 was launched upon the world the theory of evolution by natural selection, certainly one of the most powerful and far-reaching ideas ever to occur to a human mind. It occurred not to one mind, but two. Here I want to note that both Darwin and Wallace distinguished themselves not just for the discovery which they independently made, but for the generosity and humanity with which they resolved their priority in doing so. Darwin and Wallace seem to me to symbolize not just exceptional brilliance in science but the spirit of amicable cooperation which science, at its best, fosters.

  The philosopher Daniel Dennett has written: ‘Let me lay my cards on the table. If I were to give an award for the single best idea anyone has ever had, I’d give it to Darwin, ahead of Newton and Einstein and everyone else.’ I have said something similar, although I didn’t dare make the comparison with Newton and Einstein explicit. The idea we were talking about is, of course, evolution by natural selection. Not only is it the all but universally accepted explanation for the complexity and elegance of life; it is also, I strongly suspect, the only idea that in principle could provide that explanation.

  But Darwin was not the only person who thought of it. When Professor Dennett and I made our remarks, we were – certainly in my case and I suspect that Dennett would agree – using the name Darwin to stand for ‘Darwin and Wallace’. This happens to Wallace quite often, I am afraid. He tends to get a poor deal at the hands of posterity, partly through his own generous nature. It was Wallace himself who coined the word ‘Darwinism’, and he regularly referred to it as Darwin’s theory. The reason we know Darwin’s name more than Wallace’s is that Darwin went on, a year later, to publish On the Origin of Species. The Origin not only explained and advocated the Darwin/Wallace theory of natural selection as the mechanism of evolution. It also – and this had to be done at book length – set out the multifarious evidence for the fact of evolution itself.

  The drama of how Wallace’s letter arrived at Down House on 17 June 1858, casting Darwin into an agony of indecision and worry, is too well known for me to retell it. In my view the whole episode is one of the more creditable and agreeable in the history of scientific priority disputes – precisely because it wasn’t a dispute, although it so very easily could have become one. It was resolved amicably, and with heart-warming generosity on both sides, especially on Wallace’s. As Darwin later wrote in his Autobiography,

  Early in 1856 Lyell advised me to write out my views pretty fully, and I began at once to do so on a scale three or four times as extensive as that which was afterwards followed in my Origin of Species; yet it was only an abstract of the materials which I had collected, and I got through about half the work on this scale. But my plans were overthrown, for early in the summer of 1858 Mr Wallace, who was then in the Malay archipelago, sent me an essay On the Tendency of Varieties to depart indefinitely from the Original Type; and this essay contained exactly the same theory as mine. Mr Wallace expressed the wish that if I thought well of his essay, I should send it to Lyell for perusal.

  The circumstances under which I consented at the request of Lyell and Hooker to allow of an extract from my MS., together with a letter to Asa Gray, dated September 5, 1857, to be published at the same time with Wallace’s Essay, are given in the Journal of the Proceedings of the Linnean Society, 1858, p. 45. I was at first very unwilling to consent, as I thought Mr Wallace might consider my doing so unjustifiable, for I did not then know how generous and noble was his disposition. The extract from my MS. and the letter to Asa Gray…had neither been intended for publication, and were badly written. Mr Wallace’s essay, on the other hand, was admirably expressed and quite clear. Nevertheless our joint productions excited very little attention, and the only published notice of them which I can remember was by Professor Haughton of Dublin, whose verdict was that all that was new in them was false, and what was true was old. This shows how necessary it is that any new view should be explained at considerable length in order to arouse public attention.

  Darwin was over-modest about his own two papers. Both are models of the explainer’s art. Wallace’s paper is also very clearly argued. His ideas were, indeed, remarkably similar to Darwin’s own, and there is no doubt that Wallace arrived at them independently. In my opinion the Wallace paper needs to be read in conjunction with his earlier paper, published in 1855, in the Annals and Magazine of Natural History. Darwin read this paper when it came out. Indeed, it led to Wallace joining his large circle of correspondents, and to his engaging Wallace’s services as a collector. But, oddly, Darwin did not see in the 1855 paper any warning that Wallace was by then a convinced evolutionist of a very Darwinian stamp. I mean as opposed to the Lamarckian view of evolution which saw modern species as all on a ladder, changing into one another as they moved up the rungs. By contrast Wallace, in 1855, had a clear view of evolution as a branching tree, exactly like Darwin’s famous diagram which became the only illustration in The Origin of Species. The 1855 paper, however, makes no mention of natural selection or the struggle for existence.

  That was left to Wallace’s 1858 paper, the one which hit Darwin like a lightning bolt. Here, Wallace even used the phrase ‘Struggle for Existence’. Wallace devotes considerable attention to the exponential increase in numbers (another key Darwinian point). Wallace wrote:

  The greater or less fecundity of an animal is often considered to be one of the chief causes of its abundance or scarcity; but a consideration of the facts will show us that it really has little or nothing to do with the matter. Even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary, or perhaps…decreasing.

  Wallace deduced from this that:

  The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest.

  Wallace’s peroration could have been Darwin himself writing:

  The powerful retractile talons of the falcon- and the cat-tribes have not been produced or increased by the volition of those animals; but among the different varieties which occurred in the earlier and less highly organized forms of these groups, those always survived longest which had the greatest facilities for seizing their prey…Even the peculiar colours of many animals, especially insects, so closely resembling the soil or the leaves or the trunks on which they habitually reside, are explained on the same principle; for though in the course of ages varieties of many tints may have occurred, yet those races having colours best adapted to concealment from their enemies would inevitably survive the longest. We have also here an acting cause to account for that balance so often observed in nature, – a deficiency in one set of organs always being compensated by an increased development of some others – powerful wings accompanying weak feet, or great velocity making up for the absence of defensive weapons; for it has been shown that all varieties in which an unbalanced deficiency occurred could not long continue their existence. The action of this principle is exactly like that of the centrifugal governor of the steam engine, which checks and corrects any irregularities almost before they become evident.

  The image of the steam gov
ernor is a powerful one which, I can’t help feeling, Darwin might have envied.

  Historians of science have raised the suggestion that Wallace’s version of natural selection was not quite so Darwinian as Darwin himself believed. Wallace used the word ‘variety’ or ‘race’ as the level of entity at which natural selection acts. And some have suggested that Wallace, unlike Darwin who clearly saw selection as choosing among individuals, was proposing what modern theorists rightly denigrate as ‘group selection’. This would be true if, by ‘varieties’, Wallace meant geographically separated groups or races of individuals. At first I wondered about this myself. But I believe a careful reading of Wallace’s paper rules it out. I think that by ‘variety’ Wallace meant what we would nowadays call ‘genetic type’, even what a modern writer might mean by a gene. I think that, to Wallace in this paper, variety meant not local race of eagles, for example, but ‘that set of individual eagles whose talons were hereditarily sharper than usual’.

  If I am right, it is a similar misunderstanding to the one suffered by Darwin, whose use of the word ‘race’ in the subtitle of The Origin of Species is sometimes misread in support of racialism. That subtitle, or alternative title rather, is The preservation of favoured races in the struggle for life. Once again, Darwin was using ‘race’ to mean ‘that set of individuals who share a particular hereditary characteristic’, such as sharp talons, not a geographically distinct race such as the Hoodie Crow. If he had meant that, Darwin too would have been guilty of the group selection fallacy. I believe that neither Darwin nor Wallace was. And, by the same token, I do not believe that Wallace’s conception of natural selection was different from Darwin’s.

  As for the calumny that Darwin plagiarized Wallace, that is rubbish. The evidence is very clear that Darwin did think of natural selection before Wallace, although he did not initially publish. We have his abstract of 1842 and his longer essay of 1844, both of which establish his priority clearly, as did his letter to Asa Gray of 1857 which was read here on the day we are celebrating. Why he delayed so long before publishing is one of the great mysteries of the history of science. Some historians have suggested that he was afraid of the religious implications, others the political ones. Maybe he was just a perfectionist.

  When Wallace’s letter arrived, Darwin was more surprised than we moderns might think he had any right to be. He wrote to Lyell: ‘I never saw a more striking coincidence; if Wallace had had my manuscript sketch, written out in 1842, he could not have made a better short abstract of it. Even his terms now stand as Heads of my Chapters.’

  The coincidence extended to both Darwin and Wallace being inspired by Robert Malthus on population. Darwin, by his own account, was immediately inspired by Malthus’ emphasis on overpopulation and competition. He wrote in his autobiography:

  In October, 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long continuous observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work.

  Wallace’s epiphany was more delayed after his reading of Malthus, but was in a way more dramatic when it came…to his overheated brain in the midst of a malarial fever, on the island of Ternate in the Moluccas archipelago:

  I was suffering from a sharp attack of intermittent fever, and every day during the cold and succeeding hot fits had to lie down for several hours, during which time I had nothing to do but to think over any subjects then particularly interesting me…

  One day something brought to my recollection Malthus’s ‘Principles of Population’. I thought of his clear exposition of ‘the positive checks to increase’ – disease, accidents, war, and famine – which keep down the population of savage races to so much lower an average than that of more civilized peoples. It then occurred to me…

  And Wallace proceeds to his own admirably clear exposition of natural selection.

  There are other candidates for priority, apart from Darwin and Wallace. I’m not talking about the idea of evolution itself, of course; there are numerous precedents there, including Erasmus Darwin. But for natural selection there are two other Victorians who have been championed – with something like the same zeal as Baconians show when disputing the authorship of Shakespeare. The two are Patrick Matthew and Edward Blyth; and Darwin himself mentions an even earlier one, W. C. Wells. Matthew complained that Darwin had overlooked him, and Darwin subsequently did mention him in later editions of the Origin. The following is from the introduction to the fifth edition:

  In 1831 Mr Patrick Matthew published his work on ‘Naval Timber and Arboriculture’, in which he gives precisely the same view of the origin of species as that…propounded by Mr Wallace and myself in the ‘Linnean Journal’, and as that enlarged in the present volume. Unfortunately the view was given by Mr Matthew very briefly in scattered passages in an Appendix to a work on a different subject, so that it remained unnoticed until Mr Matthew himself drew attention to it in the ‘Gardener’s Chronicle’…

  As in the case of Edward Blyth, championed by Loren Eiseley, I think it is by no means clear that Matthew really did understand the importance of natural selection. The evidence is compatible with the view that these alleged predecessors of Darwin and Wallace saw natural selection as a purely negative force, weeding out misfits rather than building up the whole evolution of life (this, indeed, is a misconception under which modern creationists can be found labouring). I can’t help feeling that, if you really understood that you were sitting on one of the greatest ideas ever to occur to a human mind, you would not bury it in scattered passages in an appendix to a monograph on naval timber. Nor subsequently choose the Gardener’s Chronicle as the organ in which to claim your priority. That Wallace understood the enormous significance of what he had discovered, there is no doubt.

  Darwin and Wallace did not remain always in total agreement. In old age, Wallace dabbled in spiritualism (in spite of his venerable appearance, Darwin never reached extreme old age), and from earlier times Wallace doubted that natural selection could account for the special abilities of the human mind. But the more important conflict between them came over sexual selection, and it has ramifications to this day, as Helena Cronin has documented in her beautifully written book The Ant and the Peacock. Wallace once said of himself: ‘I am more Darwinian than Darwin himself.’ He saw natural selection as ruthlessly utilitarian and he couldn’t stomach Darwin’s sexual selection interpretation of bird of paradise tails and similar bright coloration. Darwin’s own stomach was not invulnerable. He wrote: ‘The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick.’ Nevertheless, Darwin reconciled himself to sexual selection, and became positively enthusiastic for it. Aesthetic whim, by females choosing among males, was enough to account for the peacock’s tail and similar extravagances. Wallace hated this. So did just about everybody at the time except Darwin, sometimes for frankly misogynistic reasons. To quote Helena Cronin:

  Several authorities went further, emphasising the notorious fickleness of females. According to Mivart, ‘Such is the instability of a vicious feminine caprice, that no constancy of coloration could be produced by its selective action.’ Geddes and Thomson were of the gloomily misogynistic opinion that permanence of female taste was ‘scarcely verifiable in human experience’.

  Not for misogynistic reasons, Wallace strongly felt that female whim was not a proper explanation for evolutionary change. And Cronin uses his name for an entire strand of thought which lasts to this day. ‘Wallaceans’ are biased towards utilitarian explanations of bright coloration, while ‘Darwinians’ accept female whim as an explanation. Modern Wallaceans accept that peacocks’ tails and similar bright organs are adver
tisements to females. But they want the males to be advertising genuine quality. A male with bright-coloured tail feathers is showing that he is a high-quality male. The Darwinian view of sexual selection, by contrast, is that the bright tail is valued by females for no additional qualities over and above the bright coloration itself. They like it because they like it because they like it. Females who choose attractive males have attractive sons who appeal to females of the next generation. Wallaceans more austerely insist that coloration must mean something useful.

  The late W. D. Hamilton, my colleague at Oxford University, was a prime example of a Wallacean in this sense. He believed that sexually selected ornaments were badges of good health, selected for their capacity to advertise the health of a male – bad health as well as good.

  One way to express Hamilton’s Wallacean idea is to say that selection favours females who become skilled veterinary diagnosticians. At the same time, selection favours males who make it easy for them by, in effect, growing the equivalent of conspicuous thermometers and blood-pressure meters. The long tail of a bird of paradise, for Hamilton, is an adaptation to make it easy for females to diagnose the male’s health, good or bad. An example of a good general diagnostic is a susceptibility to diarrhoea. A long dirty tail is a giveaway of ill-health. A long clean tail is the opposite. The longer the tail, the more unmistakable the badge of health, whether good health or poor. Obviously this honesty benefits the particular male only when his health is good. But Hamilton and other neo-Wallaceans have ingenious arguments*2 to the effect that natural selection favours honest badges in general, even if, in particular cases, honesty has painful consequences. Neo-Wallaceans believe that natural selection favours long tails precisely because they are an effective badge of health; both good health and (more paradoxically, but mathematical models of the theory really do stand up) poor health.