by Peter Watson
Two weeks after Arthur Evans landed in Crete, on 24 March 1900, the very week that the archaeologist was making the first of his great discoveries, Hugo de Vries, a Dutch botanist, solved a very different – and even more important – piece of the evolution jigsaw. In Mannheim he read a paper to the German Botanical Society with the title ‘The Law of Segregation of Hybrids.’
De Vries – a tall, taciturn man – had spent the previous years since 1889 experimenting with the breeding and hybridisation of plants, including such well-known flowers as asters, chrysanthemums, and violas. He told the meeting in Mannheim that as a result of his experiments he had formed the view that the character of a plant, its inheritance, was ‘built up out of definite units’; that is, for each characteristic – such as the length of the stamens or the colour of the leaves – ‘there corresponds a particular form of material bearer.’ (The German words was in fact Träger, which may also be rendered as ‘transmitter.’) And he added, most significantly, ‘There are no transitions between these elements.’ Although his language was primitive, although he was feeling his way, that night in Mannheim de Vries had identified what later came to be called genes.35 He noted, first, that certain characteristics of flowers – petal colour, for example – always occurred in one or other form but never in between. They were always white or red, say, never pink. And second, he had also identified the property of genes that we now recognise as ‘dominance’ and ‘recession,’ that some forms tend to predominate over others after these forms have been crossed (bred). This was a major discovery. Before the others present could congratulate him, however, he added something that has repercussions to this day. ‘These two propositions’, he said, referring to genes and dominance/recession, ‘were, in essentials, formulated long ago by Mendel…. They fell into oblivion, however, and were misunderstood…. This important monograph [of Mendel’s] is so rarely quoted that I myself did not become acquainted with it until I had concluded most of my experiments, and had independently deduced the above propositions.’ This was a very generous acknowledgement by de Vries. It cannot have been wholly agreeable for him to find, after more than a decade’s work, that he had been ‘scooped’ by some thirty years.36
The monograph that de Vries was referring to was ‘Experiments in Plant-Hybridisation,’ which Pater Gregor Mendel, a Benedictine monk, had read to the Brünn Society for the Study of Natural Science on a cold February evening in 1865. About forty men had attended the society that night, and this small but fairly distinguished gathering was astonished at what the rather stocky monk had to tell them, and still more so at the following month’s meeting, when he launched into a complicated account of the mathematics behind dominance and recession. Linking maths and botany in this way was regarded as distinctly odd. Mendel’s paper was published some months later in the Proceedings of the Brünn Society for the Study of Natural Science, together with an enthusiastic report, by another member of the society, of Darwin’s theory of evolution, which had been published seven years before. The Proceedings of the Brünn Society were exchanged with more than 120 other societies, with copies sent to Berlin, Vienna, London, St Petersburg, Rome, and Uppsala (this is how scientific information was disseminated in those days). But little attention was paid to Mendel’s theories.37
It appears that the world was not ready for Mendel’s approach. The basic notion of Darwin’s theory, then receiving so much attention, was the variability of species, whereas the basic tenet of Mendel was the constancy, if not of species, at least of their elements. It was only thanks to de Vries’s assiduous scouring of the available scientific literature that he found the earlier publication. No sooner had he published his paper, however, than two more botanists, at Tubingen and Vienna, reported that they also had recently rediscovered Mendel’s work. On 24 April, exactly a month after de Vries had released his results, Carl Correns published in the Reports of the German Botanical Society a ten-page account entitled ‘Gregor Mendel’s Rules Concerning the Behaviour of Racial Hybrids.’ Correns’s discoveries were very similar to those of de Vries. He too had scoured the literature – and found Mendel’s paper.38 And then in June of that same year, once more in the Reports of the German Botanical Society, there appeared over the signature of the Viennese botanist Erich Tschermak a paper entitled ‘On Deliberate Cross-Fertilisation in the Garden Pea,’ in which he arrived at substantially the same results as Correns and de Vries. Tschermak had begun his own experiments, he said, stimulated by Darwin, and he too had discovered Mendel’s paper in the Brünn Society Proceedings.39 It was an extraordinary coincidence, a chain of events that has lost none of its force as the years have passed. But of course, it is not the coincidence that chiefly matters. What matters is that the mechanism Mendel had recognised, and the others had rediscovered, filled in a major gap in what can claim to be the most influential idea of all time: Darwin’s theory of evolution.
In the walled garden of his monastery, Mendel had procured thirty-four more or less distinct varieties of peas and subjected them to two years of testing. Mendel deliberately chose a variety (some were smooth or wrinkled, yellow or green, long-stemmed or short-stemmed) because he knew that one side of each variation was dominant – smooth, yellow, or long-stemmed, for instance, rather than wrinkled, green, or short-stemmed. He knew this because when peas were crossed with themselves, the first generation were always the same as their parents. However, when he self-fertilised this first generation, or F, as it was called, to produce an F2 generation, he found that the arithmetic was revealing. What happened was that 253 plants produced 7,324 seeds. Of these, he found that 5,474 were smooth and 1,850 were wrinkled, a ratio of 2.96:1. In the case of seed colour, 258 plants produced 8,023 seeds: 6,022 yellow and 2,001 green, a ratio of 3.01:1. As he himself concluded, ‘In this generation along with the dominant traits the recessive ones appear in their full expression, and they do so in the decisively evident average proportion of 3:1, so that among the four plants of this generation three show the dominant and one the recessive character.’40 This enabled Mendel to make the profound observation that for many characteristics, the heritable quality existed in only two forms, the dominant and recessive strains, with no intermediate form. The universality of the 3:1 ratio across a number of characteristics confirmed this.* Mendel also discovered that these characteristics exist in sets, or chromosomes, which we will come to later. His figures and ideas helped explain how Darwinism, and evolution, worked. Dominant and recessive genes governed the variability of life forms, passing different characteristics on from generation to generation, and it was this variability on which natural selection exerted its influence, making it more likely that certain organisms reproduced to perpetuate their genes.
Mendel’s theories were simple and, to many scientists, beautiful. Their sheer originality meant that almost anybody who got involved in the field had a chance to make new discoveries. And that is what happened. As Ernst Mayr has written in The Growth of Biological Thought, ‘The rate at which the new findings of genetics occurred after 1900 is almost without parallel in the history of science.’41
And so, before the fledgling century was six months old, it had produced Mendelism, underpinning Darwinism, and Freudianism, both systems that presented an understanding of man in a completely different way. They had other things in common, too. Both were scientific ideas, or were presented as such, and both involved the identification of forces or entities that were hidden, inaccessible to the human eye. As such they shared these characteristics with viruses, which had been identified only two years earlier, when Friedrich Löffler and Paul Frosch had shown that foot-and-mouth disease had a viral origin. There was nothing especially new in the fact that these forces were hidden. The invention of the telescope and the microscope, the discovery of radio waves and bacteria, had introduced people to the idea that many elements of nature were beyond the normal range of the human eye or ear. What was important about Freudianism, and Mendelism, was that these discoveries appeared to be fundamenta
l, throwing a completely new light on nature, which affected everyone. The discovery of the ‘mother civilisation’ for European society added to this, reinforcing the view that religions evolved, too, meaning that one old way of understanding the world was subsumed under another, newer, more scientific approach. Such a change in the fundamentals was bound to be disturbing, but there was more to come. As the autumn of 1900 approached, yet another breakthrough was reported that added a third major realignment to our understanding of nature.
In 1900 Max Planck was forty-two. He was born into a very religious, rather academic family, and was an excellent musician. He became a scientist in spite of, rather than because of, his family. In the type of background he had, the humanities were considered a superior form of knowledge to science. His cousin, the historian Max Lenz, would jokingly refer to scientists (Naturforscher) as foresters (Naturförster). But science was Planck’s calling; he never doubted it or looked elsewhere, and by the turn of the century he was near the top of his profession, a member of the Prussian Academy and a full professor at the University of Berlin, where he was known as a prolific generator of ideas that didn’t always work out.42
Physics was in a heady flux at the turn of the century. The idea of the atom, an invisible and indivisible substance, went all the way back to classical Greece. At the beginning of the eighteenth century Isaac Newton had thought of atoms as minuscule billiard balls, hard and solid. Early-nineteenth-century chemists such as John Dalton had been forced to accept the existence of atoms as the smallest units of elements, since this was the only way they could explain chemical reactions, where one substance is converted into another, with no intermediate phase. But by the turn of the twentieth century the pace was quickening, as physicists began to experiment with the revolutionary notion that matter and energy might be different sides of the same coin. James Clerk Maxwell, a Scottish physicist who helped found the Cavendish Laboratory in Cambridge, England, had proposed in 1873 that the ‘void’ between atoms was filled with an electromagnetic field, through which energy moved at the speed of light. He also showed that light itself was a form of electromagnetic radiation. But even he thought of atoms as solid and, therefore, essentially mechanical. These were advances far more significant than anything since Newton.43
In 1887 Heinrich Hertz had discovered electric waves, or radio as it is now called, and then, in 1897, J. J. Thomson, who had followed Maxwell as director of the Cavendish, had conducted his famous experiment with a cathode ray tube. This had metal plates sealed into either end, and then the gas in the tube was sucked out, leaving a vacuum. If subsequently the metal plates were connected to a battery and a current generated, it was observed that the empty space, the vacuum inside the glass tube, glowed.44 This glow was generated from the negative plate, the cathode, and was absorbed into the positive plate, the anode.*
The production of cathode rays was itself an advance. But what were they exactly? To begin with, everyone assumed they were light. However, in the spring of 1897 Thomson pumped different gases into the tubes and at times surrounded them with magnets. By systematically manipulating conditions, he demonstrated that cathode rays were in fact infinitesimally minute particles erupting from the cathode and drawn to the anode. He found that the particles’ trajectory could be altered by an electric field and that a magnetic field shaped them into a curve. He also discovered that the particles were lighter than hydrogen atoms, the smallest known unit of matter, and exactly the same whatever the gas through which the discharge passed. Thomson had clearly identified something fundamental. This was the first experimental establishment of the particulate theory of matter.45
This particle, or ‘corpuscle,’ as Thomson called it at first, is today known as the electron. With the electron, particle physics was born, in some ways the most rigorous intellectual adventure of the twentieth century which, as we shall see, culminated in the atomic bomb. Many other particles of matter were discovered in the years ahead, but it was the very notion of particularity itself that interested Max Planck. Why did it exist? His physics professor at the University of Munich had once told him as an undergraduate that physics was ‘just about complete,’ but Planck wasn’t convinced.46 For a start, he doubted that atoms existed at all, certainly in the Newtonian/Maxwell form as hard, solid miniature billiard balls. One reason he held this view was the Second Law of Thermodynamics, conceived by Rudolf Clausius, one of Planck’s predecessors at Berlin. The First Law of Thermodynamics may be illustrated by the way Planck himself was taught it. Imagine a building worker lifting a heavy stone on to the roof of a house.47 The stone will remain in position long after it has been left there, storing energy until at some point in the future it falls back to earth. Energy, says the first law, can be neither created nor destroyed. Clausius, however, pointed out in his second law that the first law does not give the total picture. Energy is expended by the building worker as he strains to lift the stone into place, and is dissipated in the effort as heat, which among other things causes the worker to sweat. This dissipation Clausius termed ‘entropy’, and it was of fundamental importance, he said, because this energy, although it did not disappear from the universe, could never be recovered in its original form. Clausius therefore concluded that the world (and the universe) must always tend towards increasing disorder, must always add to its entropy and eventually run down. This was crucial because it implied that the universe was a one-way process; the Second Law of Thermodynamics is, in effect, a mathematical expression of time. In turn this meant that the Newton/Maxwellian notion of atoms as hard, solid billiard balls had to be wrong, for the implication of that system was that the ‘balls’ could run either way – under that system time was reversible; no allowance was made for entropy.48
In 1897, the year Thomson discovered electrons, Planck began work on the project that was to make his name. Essentially, he put together two different observations available to anyone. First, it had been known since antiquity that as a substance (iron, say) is heated, it first glows dull red, then bright red, then white. This is because longer wavelengths (of light) appear at moderate temperatures, and as temperatures rise, shorter wavelengths appear. When the material becomes white-hot, all the wavelengths are given off. Studies of even hotter bodies – stars, for example – show that in the next stage the longer wavelengths drop out, so that the colour gradually moves to the blue part of the spectrum. Planck was fascinated by this and by its link to a second mystery, the so-called black body problem. A perfectly formed black body is one that absorbs every wavelength of electromagnetic radiation equally well. Such bodies do not exist in nature, though some come close: lampblack, for instance, absorbs 98 percent of all radiation.49 According to classical physics, a black body should only emit radiation according to its temperature, and then such radiation should be emitted at every wavelength. In other words, it should only ever glow white. In Planck’s Germany there were three perfect black bodies, two of them in Berlin. The one available to Planck and his colleagues was made of porcelain and platinum and was located at the Bureau of Standards in the Charlottenburg suburb of the city.50 Experiments there showed that black bodies, when heated, behaved more or less like lumps of iron, giving off first dull red, then bright red-orange, then white light. Why?
Planck’s revolutionary idea appears to have first occurred to him around 7 October 1900. On that day he sent a postcard to his colleague Heinrich Rubens on which he had sketched an equation to explain the behaviour of radiation in a black body.51 The essence of Planck’s idea, mathematical only to begin with, was that electromagnetic radiation was not continuous, as people thought, but could only be emitted in packets of a definite size. Newton had said that energy was emitted continuously, but Planck was contradicting him. It was, he said, as if a hosepipe could spurt water only in ‘packets’ of liquid. Rubens was as excited by this idea as Planck was (and Planck was not an excitable man). By 14 December that year, when Planck addressed the Berlin Physics Society, he had worked out his full theory.
52 Part of this was the calculation of the dimensions of this small packet of energy, which Planck called h and which later became known as Planck’s constant. This, he calculated, had the value of 6.55 × 10–27 ergs each second (an erg is a small unit of energy). He explained the observation of black-body radiation by showing that while the packets of energy for any specific colour of light are the same, those for red, say, are smaller than those of yellow or green or blue. When a body is first heated, it emits packets of light with less energy. As the heat increases, the object can emit packets with greater energy. Planck had identified this very small packet as a basic indivisible building block of the universe, an ‘atom’ of radiation, which he called a ‘quantum.’ It was confirmation that nature was not a continuous process but moved in a series of extremely small jerks. Quantum physics had arrived.
Not quite. Whereas Freud’s ideas met hostility and de Vries’s rediscovery of Mendel created an explosion of experimentation, Planck’s idea was largely ignored. His problem was that so many of the theories he had come up with in the twenty years leading up to the quantum had proved wrong. So when he addressed the Berlin Physics Society with this latest theory, he was heard in polite silence, and there were no questions. It is not even clear that Planck himself was aware of the revolutionary nature of his ideas. It took four years for its importance to be grasped – and then by a man who would create his own revolution. His name was Albert Einstein.