I shall divide my discussion into three parts: that dealing with the underlying philosophical commitment to science; that dealing with the fact of evolution and that dealing with the belief that the right scientific strategy is the Darwinian one, referring here to Darwin's major mechanism of "natural selection."{1}
I take it that my position excludes certain sorts of miracles- for instance, Jesus turning the water into wine (taken in a literal sense). On the other hand, if your miracle does not interfere with the workings of nature, and if you do not make any scientific claims for your beliefs, I have nothing to say, qua my commitment to science. For instance, I see no reason why one should not be a scientist in my sense and also believe in the Catholic doctrine of transubstantiation, the belief that the water and wine turn into the body and blood of Christ during the Catholic mass. No priest would ever claim that you cut open the loaf and the flesh oozes out. The Aristotelian distinction between substance and accident lies beyond science, as I am understanding it.
Is my position reasonable, provable, irrational, or just a philosophical preference? It is certainly not provable in the sense that the theorem of Pythagoras is provable. It is not provable in the sense that one can prove that the earth goes around the sun; it is the presupposition of that particular claim! On the other hand, I deny that it is merely an irrational prejudice, or even "just" a philosophical preference. Although you must remember that I am prejudiced here, because I am a professional philosopher.
Sometimes you have beliefs which you cannot prove absolutely, but which are still reasonable in the sense that you can offer good arguments. Political beliefs probably fall into this category-but let me take the less contentious (or is it more contentious?) case of preferences about sport. Is baseball a better game than cricket? Ultimately, there is no way of deciding; some people like cricket and some people like baseball. But you can offer some reasons for your preference. For instance, a strong argument in favor of baseball is that the game gets finished. You do not go four-and-a-half days, have the game in the balance, and then get everything washed out, literally, by the English weather.
In the same way, you can offer an argument for the reasonableness of the scientific attitude. A great many things that in the past were thought miraculous are now seen to be covered by law. To take but one example, it was thought by the well-to-do in New York City in the last century that typhoid was a miraculous punishment by God of the Irish and other poor immigrants for their dissolute lifestyles and stubborn adherence to Popery. Now we know that only the bottom segment of society had to drink the stinking, contaminated water. There was no miracle about that, or about the diseases the lowest group-members thereby contracted. Of course, you can always say that there was no miracle then, but there is a genuine miracle coming up around the next corner. I say that it is not reasonable to believe this. The mature attitude is to go with unbroken law.
I should say one final thing before I move on. I recognize that having a commitment to law is one thing. It is another thing to put such a commitment into effect. I do not want to write an essay here about scientific method, but obviously what I am presuming is that the working scientist will be testing his/her hypotheses against the world of experience (which may well include the manipulated world of experience of the experimenter). Also, to guide him/her, the scientist will be relying on well-established rules of scientific method: the appeal to consistency, the preference for simplicity, the aim for comprehen- siveness.
This last dictate I take to be very important indeed, especially in the kind of case we are about to consider here. Scientists aim to include all of their subsidiary hypotheses beneath one or two major all embracing laws. The paradigmatic example of such a "consilience of inductions" was Isaac Newton's showing that the terrestrial mechanics of Galileo and the heavenly dynamics of Kepler could be subsumed beneath shared laws of motion and of gravitational attraction.
The status and proof of such rules of methodology have been matters of some debate. I myself favor an explanation that roots the rules in our evolutionary past. But that is not a significant issue here. I agree that the rules are not provable absolutely in logic; but, as with the general commitment to lawfulness, I argue that their acceptance is more than a matter of arbitrary whim. Return for a moment to the baseball analogy, although I accept that there is an element of subjectivity about the baseball case that is missing in the case of science. Even though it is true that you cannot prove absolutely that some rules of baseball are necessary, through long experience you know that unless you have rules, and rules of a certain kind, your game will cease to function in the way that you expect of a top-notch sport. If, say, a runner did not have to touch all of the bases on his way round, baseball simply would not work. The same is true of science with respect to simplicity and consilience and the like. Science will not work without rules, and experience tells us which are the best rules.
I myself do not believe any of these things. Everything I know (although, I admit candidly that I get it all at secondhand) suggests that the universe formed about twenty billion years ago, in a big bang, and that everything has been expanding and evolving ever since. But, remembering now that I take the scientific attitude for granted, why go on to accept organic evolution? Why accept that all organisms came about through a natural process of development? Why believe that we humans had monkeys for grandfathers?
If challenged with this point, many people, especially those who are not professional evolutionists, would say, "Because of the fossils." They would think that the record of the rocks is the ultimate ground for belief. Along with professional evolutionists, however, I would cast my net much wider. If you want to find out about life's history, the fossil record is invaluable. If you want to find out what our ancestors were like, then you must go and dig up the bones in Africa. Today we have many other techniques for inferring organic histories ("phylogenies"), often techniques at the molecular level, but the fossils remain crucial.
I am an evolutionist as such, because of all of the evidence. I find particularly convincing the evidence of morphology. Why is it that the limbs of vertebrates, used for all sort of different purposes, have the same isomorphic pattern of bones ("homologies")? Why do we find repetition between the forelimb of the human (a grasping instrument), the front leg of the horse (running), the flipper of the whale (Swimming), the wing of the bat (flying), and more? My answer is that if you think in terms of unbroken law, then evolution makes the most sense.
Like Darwin himself, I also find very impressive the facts of biogeographical distribution. That famous group of islands in the Pacific, the Galapagos Archipelago, has different species of finch from island to island. How could this be, other than through the gradual development from shared ancestors? If you think of seeding from outer space, or of spontaneous generation, or some such thing, there is no reason why the finches appear thus so close together. Why not one species of finch in the Galapagos and another in the Hebrides?
Am I making this claim about evolution as a matter of absolute logical necessity, given the commitment to science? The phrase I like is that of the lawyers: "Beyond reasonable doubt." I think that the fact of evolution is beyond reasonable doubt. There is no need for the student of biology to take seriously, say, the hypothesis of spontaneous generation (of whole forms). Ideas like that have been considered and discredited. You will recognize that here I am appealing to a consilience I of inductions. My claim is that evolution brings many disparate parts of biological science together and unites them beneath one all-embracing hypothesis. It is not reasonable to go on questioning.
Continuing the legal analogy, you will realize that this is precisely what we do in a court of law, especially when we are dealing with circumstantial evidence. The guilt for the murder is pinned on the butler, say, because the fingerprints and bloodstains and the broken alibi and the motive and everything else all make sense on the hypothesis of the butler's guilt. Likewise, evolution is a reasonable belief because the homologies and the biogeographical distributions and all the rest make sense on the hypothesis of the truth of evolution. The butler's guilt and the truth of evolution are "beyond reasonable doubt"
The truth of evolution is not a logical necessity: as in law, new evidence could lead one to reconsider even a verdict decided as "beyond reasonable doubt." But it is going to take a lot of evidence of a very strong nature. I am not holding my breath in anticipation. I would put the chances of my being wrong on this point about on a par with my favorite tabloid being right that Elvis is indeed alive and well and living in retirement in Florida.
My answer centers on what I think is the most important aspect of organic nature, separating it from the rest of reality. Organisms work, they function, they are as if designed; they have adaptations. It makes no sense to ask about the function of the craters on the moon. It makes every sense to ask about the function of the sail on the back of Dimetrodon. I have said that organisms are "as if" designed. Am I suggesting that they are not designed? No, nor am I saying that they are designed. My point is simply that, as one following science, if talk of a designer implies someone who got involved miraculously in the process, that idea is simply inappropriate in this context.
One must explain the adaptedness of organisms by natural means. This one can do by invoking natural selection, the differential survival of certain organisms, against a background of modern genetics. Originally this was Mendelian genetics, but now we think that ultimately all can be referred back to long chains of ribonucleic acid (DNA and RNA). I believe that, for reasons not connected with their needs, organisms vary. There is a constant pressure of population, leading to competition for resources and mates ("the struggle for existence"). Some organisms survive and reproduce. Some do not. Those that do succeed, have on average variations not possessed by the losers, and those variations, although they appeared for no good reason, helped the winners to win. Given enough time, since the variations are heritable, one gets evolution. More significant, one gets the evolution of adaptations.
The argument is simple, but not simplistic. It is true, but not a truism. It is one of the most beautiful and powerful discoveries ever made by a human being. That is why I am proud to be called a "Darwinian." It is not just that natural selection explains the world that we know already, but that like the very best scientific ideas, it contains potential for explanation in new areas of inquiry. In the last thirty years, for instance, Darwin's selection has been introduced into our study of social behavior in a major and satisfying manner. Such old problems, as for example, the structure of nests of the hymenoptera (ants, bees, and wasps), are now seen in an altogether new and revealing light.
Does this mean that I believe that natural selection is the exclusive cause of evolutionary change? If I did, I would be the first person ever to do so and I am not about to set any records here! I see no logical reason why every last aspect of the organic world should be adapted, and I very much doubt that it is. Perhaps it is the case that male nipples have some adaptive function-but I rather doubt it. Even the sexual argument does not convince. If you can learn to love people without foreskins, then I see no reason why you should not learn to love people without nipples.
It seems to me quite plausible that there are reasons of change other than direct selective advantage: correlation with other, desirable features; by-production from changes of unrelated organs; and just plain chance. This latter might be very important at the molecular level, although do not ask me how important. I am sure that natural selection is very important; I am sure that it is the most important. I just doubt that it is the exclusive cause of evolutionary change.
I have spoken in my heading to this section of Darwinism as a "strategy" and that is precisely what I mean. I think that it is true. More important, I know that it works. It explains the world we know, and it lead us into new realms of the world that we do not yet know. It makes sense as a scientist, as an evolutionist, to back it for all that it is worth. That is what sensible strategies are all about.