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haemorrhages that occurred after childbirth, and by 1875 there had been 347 recorded transfusions. But many patients were still suffering the sometimes fatal consequences of a bad reaction to the transfused blood.
    By that time, scientists were beginning to discover the differences in blood type that were causing the problem. The nature of the reaction of one blood type with another was discovered by the French physiologist Léonard Lalois when, in 1875, he mixed the blood of animals of different species. He noticed that the blood cells clumped together and frequently burst open. But it was 1900 before the biologist Karl Landsteiner worked out what was happening and discovered the first human blood group system, which divides people into Groups A, B, AB and O. When a donor’s ABO blood group matches that of the patient receiving the transfusion, there is no bad reaction; but if there is a mismatch, the cells form clumps and break open, causing a severe reaction. There is some historical evidence that the Incas of South America had practised transfusions successfully. Since we now know that most native South Americans have the same blood group (Group O), the Inca transfusions would have been much less dangerous than attempts in Europe, because there was an excellent chance that both donor and patient would belong to Group O and thus be perfectly matched.
    Unlike the complicated genetics which governs the inheritance of hair, which is still not fully understood, the rules for inheriting the ABO blood groups turned out to be very simple indeed. Precisely because the genetics were so straightforward and could be followed easily from parents to offspring, blood groups were widely used in cases of contested paternity until recently, when they were eclipsed by the much greater precision of genetic fingerprints. Their significance for our story in this book is that it was the blood groups which first launched genetics on to the world stage of human evolution. For this debut we need to go back to the First World War and to a scientific paper delivered to the Salonika Medical Society on 5 June 1918. It was translated and published the following year in the leading British medical periodical The Lancet under the title ‘Serological differences between the blood of different races: the results of research on the Macedonian Front’. To give you a flavour of the sort of thing The Lancet published in those days, the article was sandwiched between a discourse by the eminent surgeon Sir John Bland-Sutton on the third eyelid of reptiles and a War Office announcement that those nurses who had been mentioned in dispatches for their work in Egypt and France would soon be getting a certificate from the King showing his appreciation.
    The authors of the blood group paper were a husband and wife team, Ludwik and Hanka Herschfeld, who worked at the central blood group testing laboratory of the Royal Serbian Army, which was part of the Allied force fighting against the Germans. The First World War had a great influence on bringing blood transfusion practice towards its modern standards. Before the war it had been customary for physicians with a patient who needed a transfusion to test the blood groups of friends and relatives until a match was found, then bleed the donor and immediately give the blood to the patient. The high demand for transfusions on the battlefields of Europe meant that ways had to be found to store donated blood in blood banks ready for immediate use. All soldiers had their blood group tested and recorded so that, should they need an urgent transfusion to treat a serious battlefield wound, compatible blood of the correct type could be immediately drawn from the blood bank.
    Ludwik Herschfeld had already demonstrated, some years earlier, that blood groups A and B followed the basic genetic rules laid out by Gregor Mendel. He was not sure what to make of blood group O and set it aside, though it was later shown to obey