The Causes of Evolution

J.B.S. Haldane

Longmans: London, 1932.

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Haldane's popular overview of his mathematical theory reconciling Mendel with Natural Selection. For the technical details, see this complete collection of all the original series of papers by Haldane on 'The Mathematical Theory of Natural and Artificial Selection'.

This book is based on a series of lectures delivered in January 1931 at the Prifysgol Cymru, Aberystwyth, and entitled "A Re-examination of Darwinism." These lectures were endowed by the munificence of the Davies family, with the provision that their substance should be published in book form. This admirable condition ensures that, unlike the average university lectures, which stale with great rapidity, they should only be delivered once, and also that they should be made generally available before any novelty which they may possess has worn off.

Apart from the Appendix, I have added very little to the lectures as delivered. I doubt whether the time is yet ripe for a really comprehensive book covering the same ground, because our knowledge of the cytological nature of differences between species is increasing so rapidly as to render any account of these differences very provisional.

Readers who are not versed in biology will be well advised to skim lightly over Chapters II and III, which summarise our knowledge of those branches of genetics which are most important to the book's argument. I fear that the mathematical appendix will have a limited appeal. But I venture to hope that certain arguments in the body of the book (in particular that which purports to prove that mutation, Lamarckian transformation, and so on, cannot prevail against natural selection of even moderate intensity) will not be rejected unless a fallacy is discovered in the mathematical reasoning on which they rest.

I have to thank my colleagues of the John Innes Horticultural Institution, not only for permission to mention their unpublished work, but for many of the ideas which are here presented. Finally I wish to record here the very pleasant memories which I preserve of the week during which I had the honour to be a member of the staff of the National University of Wales.

Seventy-two years have now elapsed since Darwin and Wallace (1858) formulated the theory that evolution had occurred largely as a result of natural selection. The doctrine of evolution was not, of course, new. But Lamarck and other eminent biologists had failed to convince the scientific world or the general public that evolution had occurred, still less that it had occurred owing to the operation of any particular set of causes. Darwin contrived to carry a considerable measure of conviction on both these points. The result has been that a generation ago most people who believed in evolution held that it had been largely due to natural selection. Nowadays a certain number of believers in evolution do not regard natural selection as a cause of it, but I think that in general the two beliefs still go together.

So close a correlation is rather rare in the history of human thought. For example, men had been aware for ages of the existence of a past history of the human race before Daniel (or the author of the Book of Daniel) made the first attempt to view that history as a whole, and give a summary account of it. If Daniel had been the first person to persuade thinking men that the past had differed appreciably from the present, it is clear that his particular account of the historical process would have had a greater intellectual influence than it has actually had. We must therefore carefully distinguish between two quite different doctrines which Darwin popularised, the doctrine of evolution, and that of natural selection. It is quite possible to hold the first and not the second. Similarly with regard to the doctrines of Darwin's great contemporary Marx, it is possible to adopt socialism but not historical materialism.

Darwinism has been a subject of embittered controversy ever since its inception. The period up till Darwin's death saw a vast mass of criticism. This was mostly an attack on the doctrine of evolution, and was almost entirely devoid of scientific value. The few really pertinent attacks were lost amid a jabber of ecclesiastical bombinations. The criticism was largely dictated by disgust or fear of this doctrine, and it was natural that the majority of scientific men rallied to Darwin's support. By the time of Darwin's death in 1882, Darwinism had become orthodox in biological circles. The next generation saw the beginnings of a more critical attitude among biologists. It was possible to criticise Darwin without being supposed to be supporting the literal authenticity of the Book of Genesis. The criticism came from all sides. Palaeontologists, geneticists, embryologists, psychologists, and others, found flaws of a more or less serious character in Darwin's statements. But they almost universally accepted evolution as a fact.

The rising generation of biologists, to which I belong, may now perhaps claim to make its voice heard. We have this advantage at least over our predecessors, that we get no thrill from attacking either theological or biological orthodoxy; for eminent theologians have accepted evolution and eminent biologists denied natural selection.

In this course of lectures I do not propose to argue the case for evolution, which I regard as being quite as well proven as most other historical facts, but to discuss its possible causes, which are certainly debatable. It will, however, be worth while briefly to explain what is meant by evolution, and to indicate the arguments which lead the overwhelming majority of biologists to believe in it.

By evolution we mean the descent from living beings in the past of other widely different living beings. How wide the difference must be before the process deserves the name of evolution is a doubtful question. Many would refuse to dignify the changes which man has effected in the dog as evolution, though they have certainly an obvious bearing on the question of evolution. In the first generation after Darwin it was pointed out that artificially produced races, if they were incapable of breeding together, were so on mechanical grounds only, and never gave sterile hybrids like the mule. Since then races which, like species, are sterile on physiological grounds, or which give sterile hybrids, have been artificially produced ; but those who have produced them are chary of claiming that they have originated a new species.

Certain of the critics of evolution have admitted the possibility of fairly large structural or functional changes, but not of such a profound change as the origin of consciousness or reason. I sympathise with their attitude, but cannot share it, because it seems to me to rest on a refusal to face certain per- fectly amazing facts of everyday life. The strangest thing about the origin of consciousness from unconsciousness is not that it has happened once in the remote past, but that it happens in the life of every one of us. An early human embryo without nervous system or sense organs, and no occupation but growth, has no more claim to consciousness than a plant—far less than a jelly-fish. A new-born baby may be conscious, but has less title to rationality than a dog or ape. The evolutionist makes the very modest claim that an increase in rationality such as every normal child shows in its lifetime has occurred in the ancestors of the human race in the last few million years. He does not claim to be able to explain this process adequately, or even to understand it. But he claims that such an increase in rationality is a fact of everyday experienced It is conceivable, though to my mind unlikely, that there was a sharp break at some point in human evolution at which a new type of mental activity suddenly became possible. But there is a vastly greater probability of finding evidence of such a discontinuity in individual than in racial history. I do not think the likelihood very great, but if I believed in such radical changes, that is where I should be inclined to look for them.