The remarkable development of Metschnikoff’s doctrine of phagocytosis during the past quarter of a century is certainly one of the characteristic features of the activity of biological science in that period. At first ridiculed as ‘Metschnikoffism,’ it has now won the support of its former adversaries. For a long time the ideal of hygienists has been to preserve man from all contact with the germs of infection, to destroy them and destroy the animals conveying them, such as rats, mosquitoes, and other flies. But it has now been borne in upon us that, useful as such attempts are, and great as is the improvement in human conditions which can thus be effected, yet we cannot hope for any really complete or satisfactory realisation of the ideal of escape from contact with infective germs. The task is beyond human powers. The conviction has now been arrived at that, whilst we must take every precaution to diminish infection, yet our ultimate safety must come from within--namely, from the activity, the trained, stimulated, and carefully guarded activity, of those wonderful colourless amœba-like corpuscles whose use was so long unrecognized, but has now been made clear by the patiently continued experiments and arguments of Metschnikoff, who has named them ‘phagocytes.’ The doctrine of the activity and immense importance of these corpuscles of the living body which form part of the all-pervading connective tissues and float also in the blood, is in its nature and inception opposed to what are called the ‘humoral’ and ‘vitalistic’ theories of resistance to infection. Of this kind were the beliefs that the _liquids_ of the living body have an inherent and somewhat vague power of resisting infective germs, and even that the mere living quality of the tissues was in some unknown way antagonistic to foreign intrusive disease-germs. [Illustration: FIG. 37. FIG. 38. FIG. 39. Fig. 37.--Phagocyte or colourless corpuscle of a guinea-pig in the act of engulphing two Spirilla or parasitic vegetable microbes of a spiral shape. Fig. 38.--The same half an hour later, one of the Spirilla is nearly completely engulphed. Fig. 39.--The same ten minutes later still, one of the Spirilla is completely absorbed into the substance (protoplasm) of the phagocyte. (_From_ Metschnikoff’s book, “Immunity,” kindly supplied by the Cambridge University Press.)]