Yet, Behring's discovery of antitoxins still hung over it like a sword of Damocles; it was imperative that the respective parts played by antitoxins and by phagocytes should be elucidated. With that object in view, Metchnikoff undertook new researches and succeeded in ascertaining once for all the narrow link between immunity and the function of the phagocytes which probably elaborate the antitoxins as a product of their digestion of vaccinal toxins. He drew this conclusion from the fact that, in a rabbit vaccinated against hog-cholera, the exudate devoid of phagocytes[20] is neither bactericidal, nor antitoxic, nor attenuating, while it is so if it contains phagocytes. Therefore a relation of causality exists between cells and the acquired properties of humors. And the resistance of the animal is in visible correlation with the degree of phagocytosis which is manifested by it. [20] Aqueous humor, the exudate of aseptic oedemata. These results having been established, it seemed as if the last rampart of the humoral theory had been taken by storm. In the meanwhile the persistent and bitter opposition of physicians to the phagocyte theory made a great impression on Metchnikoff, and, while stimulating his energy in defence of his ideas, it maintained him in a state of nervous excitement and even depressed him. He asked himself why this obstinate opposition to a doctrine based on well-established facts, easily tested and observed throughout the whole animal kingdom? To him, a naturalist, it seemed clear and simple and all the more admissible that it was confirmed by the generality of its application to all living beings. But, he thought, perhaps the real cause of the attitude of the contradictors lies in the very fact that medical science only concerns itself with the pathological phenomena of higher animals, leaving their evolution entirely out of account, as well as their starting-point in lower animals--whilst it is the very simplicity of the latter which allows us to penetrate to the origin of the phenomena.