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The Mendel-Darwin Connection

When Charles Darwin published his Origin of Species in 1859, it sparked a huge controversy.  Most famous was the clash between Darwin’s friend and self-styled “bulldog,” the anatomist Thomas Henry Huxley (grandfather of the novelist Aldous Huxley), and the Bishop of Oxford, Samuel Wilberforce (son of William Wilberforce of abolition-of-slavery fame). Supposedly, Wilberforce asked Huxley if he was descended from monkeys on his grandfather’s side or on his grandmother’s side. Supposedly, Huxley replied that he would rather be descended from a monkey than from a bishop of the Church of England. As it happens with most good stories (like Moses and the flight from Egypt) the encounter was probably nothing like as dramatic as history reports; although, also as with most good stories (like Moses and the flight from Egypt) the myth has great meaning for later generations, shoring up beliefs and making people feel good about the situation in which they now find themselves.

Darwin was not particularly worried about the religious criticisms and in fact for every clerical opponent there was a clerical supporter. The Reverend Baden Powell (father of the founder of scouting) for instance was an instant and enthusiastic spokesman for Darwin’s ideas. What did worry Darwin were the scientific criticisms.  Two in particular were very troublesome. The first was the age of the earth. The physicists, ignorant as they were of the warming effects of radioactive decay, put the age at about a hundred million years, far too short a time thought Darwin for so leisurely a mechanism as natural selection. There was not much he could do about this objection, although he did make more and more use of the inheritance of acquired characteristics (so-called Lamarckism) thinking this would speed things up somewhat. 

The second big problem was the nature of heredity. If natural selection or any other mechanism was to work and have lasting effects, then the gains had to be conserved and passed on from one generation to the next.  Regrettably, not only did Darwin have no good ideas on this subject, but he got hold of the wrong end of the stick. When two organisms mate, then there are basically two possible outcomes. Their differences can as it were blend in the next generation. Human skin color is a good example. Our president is about half way between his dad and his mom. Or they can stay separate. Our president is a male like his dad and not female like his mom. Darwin of course knew about the two possibilities but he assumed that the blending option is the norm and the separate option needs special explanation. Unfortunately, as critics pointed out, blending means that however good a new feature may be, in a generation or two it will be blended to virtual nonbeing. Without massive amounts of new variation, something no one really believed in, evolution is really going to go nowhere.

As we all know today, unknown to Darwin, in his monastery garden the Moravian monk Gregor Mendel was working away developing just the mechanism of heredity that was needed by the theory of the Origin. Mendel experimenting on pea plants was showing that the basic mechanism of heredity is nonblending, and that in fact a process like natural selection that picks out good new variations can be fully effective as a means of evolutionary change. Mendel provided the famous ratios (of variations passed on from one generation to the next) that are the basis for what today we know as “Mendel’s laws.”  The trouble is that, as we also all know today, Darwin never read Mendel and so his problem went unsolved. Indeed, it was not until the next century (around 1900), when new researchers worked with Mendel’s discoveries, that natural selection and heredity could be brought harmoniously together and the route was then opened for the neo-Darwinian theory of evolution that is the dominant paradigm in biology today.

As it happens, Mendel’s work is back in the news.  It was long thought that the original manuscript that he wrote, telling of his pea-plant work and of his revolutionary discoveries, had been lost. It has turned up again and now is at the center of a tug of war between Mendel’s relatives (through his sisters) and the Augustinian order to which he belonged. Frankly, I don’t much care who ends up getting it. What counts are the ideas and these were in the public domain as soon as Mendel published. However, as a historian and philosopher of science I do rather care about how we regard the ideas of the manuscript, and here I think there is room for misunderstanding. Certainly the New York Times article on the manuscript (by veteran science reporter Nicholas Wade) gives a somewhat misleading impression.

Wade makes the claim that, in importance, the Mendel manuscript (at least, the work that it incorporates) is second in the history of modern biology to the Origin itself. I am not at all sure about that. I would have thought the discovery of cell theory (by Theodor Schwann and Matthias Jakob Schleiden in 1839) rates pretty highly, and that the Watson-Crick paper on the double helical structure of DNA in 1953 has a good claim to the prize. But who came second is not the real issue and we can in any case be glad that the manuscript (and more importantly the ideas it incorporated) do still exist. What really worries me about the claim, and what is made even more explicit by Wade later in his discussion, is the implication that Mendel really was trying to plug the gap in Darwin’s theory. What a pity therefore that Darwin never got his hands on the paper and read it or that Mendel never wrote to Darwin and explained what he was up to.

I fully agree (and have said as much above) that there are ideas in Mendel’s paper that, when developed, do plug the gap in Darwin’s theory, but this is not quite the same thing. What we do know is that, contra Wade, had Darwin read Mendel’s paper he would have been unimpressed. He thought that the key to evolutionary change is very small variations. Big variations take organisms out of adaptive focus and are worthless. But Mendel did work on big variations and Darwin thought these irrelevant to his problem. What we also do know is that Mendel never thought of himself as working on Darwin’s problem and as trying to solve it. Rather he was working on some rather technical problems about breeding that German botanists had raised and his famous ratios came out as part and parcel of this. Darwin may not have read Mendel but Mendel read Darwin!  As soon as the Origin was published in German, Mendel got a copy and read it carefully, putting notes in the margin. But Mendel’s notes were never along the lines of “I can solve Herr Darwin’s problems.” They were more predictable along the lines of “Can I a Catholic priest accept evolution?” (He found that he could.)

The historical fact is that, when some 30 or 40 years later the Mendelian ratios were rediscovered, the understanding of heredity and its underlying causes had advanced greatly, and by then people were in a position to recognize and appreciate the worth of Mendel’s discoveries—a worth that would have surprised Father Mendel greatly. And so without detracting from Mendel’s work, we should be careful not to make more of it than we do. The moral of the story is that we should beware of taking the knowledge that we have today and reading it straight back into the past. What we might think of as missed opportunities might not have at all seemed that way to the folk back then and we should be careful of judging them in our terms rather than theirs. As the novelist L.P. Hartley said at the beginning of The Go-Between: “The past is a foreign country: they do things differently there.”

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