In the 1890's, when immunology was beginning to develop, there was no difficulty in fitting it into the Darwinian pattern of survival of the fittest. The phagocytes in Elie Metchnikoff's theory of immunity and the serum antibodies in the theory of Emil von Behring both had an obvious role in the defense of the organism against attacking bacteria. Though Darwinism no longer allowed biologists to assume the design of parts for a purpose, it became almost impossible not to think in terms of their usefulness in the struggle for existence, particularly where the usefulness was as clear as was the case with the mechanisms of immunity. The resistance of some species to infections which cause disease in others was a little more difficult to fit into this pattern than was the immunity which followed the invasion and defeat of bactena, but von Behring, in an essay of 1899 on the history and theory of immunity, placed the problem generally "within the domain of the hypothesis developed by Darwin on variety, selection, adaptation and the laws of inheritance."' Emile Duclaux, director of the Institut Pasteur in Paris, writing in 1896, saw the phagocytic white cells as repulsing a foreign enemy: "the white cells of the blood play the role of gendarmes in the organism, circulating constantly, always ready to throw themselves on anything that appears to be foreign, that is, an enemy, living or dead."2 A few years later, in 1899, he was still using the same imagery of armed struggle: