The Evolution of a Proof

Although the idea of evolution was widely current before the publication of The Origin of Species, Darwin’s having propounded a theory of how evolution might have come about added enormously to its credibility and the scale of its threat to settled opinion. The reception of Darwinism by the lay press was already known from Alvar Ellegård’s admirable and enthralling Darwin and the General Reader,¹ from which we learn that the contentious element in the struggle to establish Darwinian theory was not its scientific credentials so much as its threat to right-thinking opinion. We also learn with shame how very quickly Darwinism was made to serve the cause of racism. If Negroes were not a different species from white Europeans then they represented an intermediate stage between them and the remotely ancestral apes. Opinions of the same intellectual stature as this are still to be heard today.

Hull’s learned and deeply reasoned book, which is likely to find a permanent place in the library of the history of ideas, surveys the scene from the lofty standpoint of methodology and the philosophy of science. He shows how thoroughly the reception of Darwin’s theory was obfuscated by the widespread illusion that theories such as his could be conclusively “proved” and that Darwin himself should have provided such a proof. No technical idea is so grievously misused in common speech as that of proof. People tend to use the word “proof” in the empirical sciences as if it had the same weight and connotation as it has in logic and mathematics, for we do indeed in the strictest sense of the word prove Euclid’s theorems to be true by showing that they follow deductively from his axioms and postulates. But with the empirical sciences and especially with ideas of the generality of evolution, gravitation, and even the roundness of the earth, it is not so much a question of finding “proofs” as of expounding the grounds for having confidence in them.

I can remember from my earliest schooldays being told that the earth was round and being given a number of proofs that this was indeed the case: I was told that if I saw a ship coming over the horizon I should see first the tip of the mast, then the mast as a whole, and then the bow, until finally the whole ship hove into view. I remember also being told that if three sticks were stuck into the ground some distance apart and the same distance above the ground, I should find by casting my eye along them that the middle one would look higher than the outer two.

Even as a child (doubtless a very irritating one) I can remember thinking that these proofs were pretty feeble considering the importance and unfamiliarity of the idea they professed to sustain, and so it is with the idea of evolution. For one is taught in one’s first biology classes that there are a certain number of proofs of evolution—the palaeontological, the embryological, the comparative-anatomical, and so on. But just as the idea of the sphericality of the earth does not rest upon the testimony of a number of decisive “proofs,” but rather upon the fact that the notion is implicit in the whole of chronology, geodesy, and aerial and nautical navigation, so also the acceptance of the theory of evolution rests upon the fact that it alone explains and makes sense of the variety of living forms, their succession in time, their patterns of development, and the relationships among their constituent parts.

In short, it would be a mind-blowing exercise to cease to believe in evolution or the sphericality of the earth. If there have been any “proofs” of evolution they have come from great comparative anatomists like Gegenbaur, Goodrich, and van Wijhe, and from embryologists like von Baer and Wilhelm Vogt; and to these should be added, in a quite different category, the microbial geneticists who have shown that the so-called “adaptation” of bacteria to resist the action of drugs such as penicillin is in reality an evolutionary process.

The history of the degree of esteem in which Darwinism has been held since his own day is specially interesting. Certainly there was a period lasting from perhaps 1890 to 1920 during which many experienced natural historians (d’Arcy Thompson was certainly one)² were getting rather fed up with the apparently illimitable explanatory glibness of Darwinism: it was almost impossible even to imagine any evolutionary phenomenon that could not be accounted for by the same smooth form of words about the selection of favorable variations occurring in every direction. Critically minded people feel much the same today about the pretensions of Freudian psychoanalytic theory. However that may be, every biologist competent to express an opinion now recognizes natural selection as the principal agency of evolutionary change.

Although it has been filled in and filled out, Darwin’s theory remains substantially true and his own reputation has never been higher. All biologists except the very young who are in too much of a hurry for their own good read The Origin of Species, and all may still marvel at its closeness of reasoning, cogency, and (for those who go by style) its unmistakable air of authenticity. It would be a terrible shame if racists disguised as eugenic cranks and the latter disguised as practitioners of some other quite unrelated science (e.g., physics) were now to diminish the esteem in which Darwin is so rightly held.

There is a paragraph in Hull which suggests that he finds Darwinian theory still at fault from a strictly methodological point of view. Darwinians have yet to produce a theory which makes specific predictions possible. I think the justice of this criticism really depends on how specific the predictions have to be. Let us imagine, as is not improbable, that a metabolic product which we shall call gorgonzolin of the mold Penicillium gorgonzoli is a powerful antibiotic. From what one knows of the genetic system of bacteria it is already quite possible to predict that if strains of streptococci or staphylococci are cultivated in sublethal concentrations of gorgonzolin a new variant will eventually appear which is entirely resistant to the action of gorgonzolin. It is true that gorgonzolin has not yet been discovered, but a great many fungal antibiotics have been, and I predict that what has turned out to be true of all of them would turn out to be true of gorgonzolin also.

One of the most interesting exercises made possible by Hull’s admirable text is to see how the professional philosophers and methodologists of science faced Darwinism. Many of his great admirers will be pleased to know that C. S. Peirce comes out splendidly, for not only did he describe Darwin’s hypothesis as “one of the most ingenious and pretty ever devised,” but also—and much more important—he recognized at once what it took most Darwinians something like fifty years to discover themselves: Darwin’s theory of evolution by natural selection was essentially statistical and based on population-dynamics, i.e., was Maxwellian rather than Newtonian in theoretical structure. The reason Darwinians took at least fifty years to make this discovery for themselves was, as G. G. Simpson has pointed out, because of a typological or “dynastic” habit of thinking of the evolutionary process as one in which one ruling organism succeeded another much as one monarch succeeded another on European thrones, the whole process being set out in a family tree.

In spite of his clear grasp of what we should nowadays call the “hypothetico-deductive method,” William Whewell’s appraisal of Darwin was disappointing, and one cannot help the uneasy feeling that such a prominent pillar of the establishment as the Master of Trinity College, Cambridge, would have thought it advisable to be seen to be on good terms with God. Sir John Herschel was a very influential writer on methodology in his day; he influenced Charles Darwin and, indeed, fired him with enthusiasm for the idea of adding still further to the majestic structure of science. But Herschel, too, had a God to placate: Francis Bacon, the awe-inspiring philosopher of the scientific revolution. Perhaps John Stuart Mill comes out best; but although he endorses the legitimacy of Darwin’s procedure in following the “Logic of Discovery” he chides Darwin, as everybody else did, for having failed to provide proof.

One of the strengths of Hull’s book is that he writes learnedly and in some detail on the logic of scientific method. On some subjects, for example the shortcomings of the pyramidal model of scientific structure in which axioms are seen as the tip of the pyramid and a multitude of particulars form its base, he is particularly good. The only thing that a critic determined to find fault as an earnest of impartiality would find to object to in Hull is that his prose style is a little heavy. He needs to be a little more pétillant, not in spite of but because of the gravity of the subject with which he deals.