writings include some 150 articles in professional journals. After the establishment of the Molteno Institute, Nuttall became increasingly occupied with administration and fundraising, and published few papers. His wife Paula died in 1922. One publication from this period is a biographical note on German scientists of an earlier generation who he had known. He resigned the Quick Professorshipin 1931, and became Emeritus Professor of Biology. He died suddenly in December 1937, and was survived by two sons, George and Winfred, and a daughter Carmelita. His ashes were buried in Tunbridge Wells, Kent. Mechanisms of Immunity In the early 1880s, Metchnikoff had observed phagocytosis of bacteria and other foreign matter by animal cells including leucocytes in mammals, and proposed that this was the mechanism by which animals protect themselves against infection. This idea was controversial at the time. Josef von Fodor had shown an apppparent germicidal action of blood against anthrax bacilli, but critics suggested that the bacilli were being entrapped by clotting rather than destroyed. Nuttall, carried out a series of experiments with defibrinated blood from various species, and clearly established the germicidal action of blood in the absence of clotting, and also that the germicidal action was lost after the blood was heated to 55 °C. These results formed the basis of the humoral theory of immunity, as opposed to the cellular theory of Metchnikoff, and also stimulated the work that led to the development of antitoxin therapy, particularly for diphtheria and tetanus. Initially the humoral and cellular theories were seen as rival explanations of immunity, but it soon became clear that processes of both kinds occur and complement one another. Phylogenetic relationships Darwin's theory of Evolution - descent with modification - provided a theoretical basis for classification in biology, species within a taxon sharing a more or less recent common ancestor. For nearly half a century, the only criteria for classification remained morphological, as had been the case with pre-evolutionary systems such as that of Linnaeus. Classification based on morphology has two main limitations: difficulty in dealing with convergent evolution where similar forms arise in species that are not closely related; and an inability to provide a quantitative measure of relatedness. The precipitin reaction was first described by Rudolf Kraus in 1897, an insoluble product being formed in an antigen-antibody interaction. The reaction was at first thought to be specific, but it was soon found that while the