In Britain, the 1930s was a time of revival for the romantic love of nature and the old country way of life in particular. There was a surge of people reading Gilbert White’s eighteenth century letters from Selborne, and an increase in the English enjoyment of rambling and country pursuits, suggesting a growing respect for the environment. Railway posters showed what the countryside offered the traveller, architects designed landscaped towns and artists like John Piper and Paul Nash were popular.

(Tate Gallery)                                                       (www.pinterest.com)

Although biogeography was documented in the early days of European Empire, and it was shown to be a powerful tool in evolutionary studies by Wallace and Darwin, little happened in the following fifty years apart from data-gathering. It was so difficult to map the ranges of animal and plant species and not until after that did species begin to get mapped and migrations tracked. One of the pioneers was an academic geographer Ronald Good who spent much of his time surveying the distribution of plant groups both from the literature and from his field observations. To start with the work was on a crude scale and inevitably showed the need to make smaller scale maps, and Good began to prepare these for the plant cover in his home county of Dorset. Arthur Tansley, by then returned to work in plant ecology, also began to devise ways of mapping biodiversity, but lacked resources and methods for any useful detail.

Still looking for conclusive evidence to prove his theory of continental drift, Wegener made his third and final expedition to Greenland in 1930. He had compared a series of longitudinal measurements on the island which changed in respect to the Greenwich meridian, to suggest it was drifting westwards by a few centimeters a year away from north-west Europe. He also found published records of more fossils from there that were from the same extinct species found in Spitsbergen, Scotland and Arctic Canada, locations that were later shown to include giant redwood forests.

Progress on Wegener’s other biogeographical front had been made in the 1930s by a South African geologist, Alexander Du Toit, who had found the same historical sequence of 250 million year old Devonian fossils in Argentina as those on the other side of the Atlantic in South Africa. Jaws of 200 million year old Triassic reptiles were also discovered at about the same time in Antarctica and South Africa, the same age as a peculiar fossil plant that was also found in South America, Australasia and India. It was the first suggestion of an ancient continent called Gondwanaland that existed before these modern land masses broke up about 190 million years ago. Eduard Suess had named that continent back in 1861 when he suggested that the tongue-shaped Glossopteris leaves he had described from these southern areas had been from part of the same continent. Not thinking that it might have broken up and drifted apart, he suggested that the ancient land-mass had been flooded to form the present shore lines.

Despite these and more convincing discoveries by palaeontologists in the 1930s most other geologists refused to accept the outrageous theory that continents had drifted apart. They needed hard quantitative evidence from the physical sciences before they could accept an explanation for most things and it was to take another world war to push forward the technology that eventually came up with the evidence that impressed them. Meanwhile it was for Julian Huxley to popularise the idea of “living fossils” citing examples such as the lungfish, the mollusc Lingula, the fossil conifer Metasequoia that had just been found growing in China and the earlier simple-seed tree Ginkgo. These were the real-life relicts of quiet and unchanged times, surviving in small ecological niches that had remained much the same for millions of years. They were niches that somehow survived through times of environmental change or maybe they were species that could tolerate the different conditions. In most cases, these single species had many close relatives now extinct. They are the sole survivors of old competitions.

It was during such an interval of quiet reflection that a useful explanation of how genetics works came from an unexpected conversation between GH Hardy the mathematician and the new professor of genetics Reg Punnett. These two spectators were sitting next to one another at the Cambridge cricket ground when rain had stopped play.

Their conversation turned to mathematics and genetics and Punnett let slip that no-one had answered the difficult question about why genetically dominant characters didn’t just go on increasing within a population. It was a good question, Punnett thought, and it expressed a lot of the doubt about Mendel’s work that still remained; if the ratios between dominant and recessive features showed a clear pattern then genes really did have a big role to play in genetics. Hardy took on this “very simple point” and was “surprised that the answer was not familiar to biologists.” He explained that such a population kept its genetic properties stable and his calculations became one of the central principles of Mendelian genetics, the Hardy-Weinberg Law, proving that gene frequencies reached equilibrium in one generation of random mating.

While reflecting on his very difficult experiences during the war Haldane maintained his optimism, reassuring some critics that science had made warfare less horrible than before. Bombs were more merciful than bayonets and he thought that gas was the most humane of the three. What did upset him was the convention of defending the front lines with penniless men from the Empire and others from the poorest homes in Britain and it caused him to become involved with some of the new political societies that began after the war. It also caused Haldane to disagree with his friend GH Hardy that mathematics should be encouraged for its many applications in war, especially gunnery and aircraft design. This certainly was why many of the hard sciences flourished between the wars, providing the armaments industry with new opportunities.

Haldane took these controversial applications to heart and decided to switch his career back to his pre-war interests in mutation. In 1922 he became Reader in Biochemistry at Cambridge, hoping to show that natural selection could not only cause a species to change, but also that it could do so at a rate which accounted for present and past transmutations. It was an ambitious scheme with so little data and he spent a lot of time working out how many generations were needed for change to spread through populations.

The work was theoretical and his old friend at Oxford, the colourful butterfly expert EB Ford, encouraged him to bring data from the famous peppered moth into his calculations.

Pale grey moths had been common in English woodlands for centuries, well camouflaged from their predators when they fed from tree trunks. The darker form was much rarer and was kept down in these small numbers by hungry birds. As industrial pollution darkened the tree trunks through the years of the industrial revolution, the birds found it easier to spot the pale grey forms and so the balance changed. Haldane’s calculations showed a great intensity of natural selection in favour of the dark colour, and for the species to survive there needed to be 50% more specimens. This led to Haldane’s Principle in 1937, a rule that stated that harmful mutations would usually go unnoticed if they fell below a certain level.