should behave in the same way and pour out sensitisers and opsonins to paralyse, and prepare their bacterial food. And the experiments of Metschnikoff’s pupils and followers show that this is undoubtedly the case. Whether there is any great variety of and difference between ‘sensitisers’ and ‘opsonins’ is a matter which is still the subject of active experiment. Metschnikoff’s conclusion, as recently stated in regard to the whole progress of this subject, is that the phagocytes in our bodies should be stimulated in their activity in order successfully to fight the germs of infection. Alcohol, opium, and even quinine, hinder the phagocytic action; they should therefore be entirely eschewed or used only with great caution where their other and valuable properties are urgently needed. It appears that the injection of blood-serum into the tissues of animals causes an increase in the number and activity of the phagocytes, and thus an increase in their resistance towards pathogenic germs. Thus Durham (who was a pioneer in his observations on the curious phenomena of the ‘agglutination’ of blood corpuscles in relation to disease) was led to suggest the injection of sera during surgical operations, and experiments recently quoted by Metschnikoff seem to show that the suggestion was well founded. Both German and French surgeons have employed the method with successful results, and the demonstration that an immense number of microbes are thus taken up and destroyed by the multiplication (due to their regular increase by cell-division) of the phagocytes of the injected patient. After years of opposition bravely met in the pure scientific spirit of renewed experiment and demonstration, Metschnikoff is at last able to say that the foundation-stone of the hygiene of the tissues--the thesis that our phagocytes are our arms of defence against infective germs--has been generally accepted. Another feature of the progress of our knowledge of disease--as a scientific problem--is the recent recognition that minute animal parasites of that low degree of unicellular structure to which the name ‘Protozoa’ is given, are the causes of serious and ravaging diseases, and that the minute algoid plants, the bacteria, are not alone in possession of this field of activity. It was Laveran--a French medical man--who, just about twenty-five years ago, discovered the minute animal organism in the red blood-corpuscles, which is the cause of malaria (see fig. 44). Year by year ever since our knowledge of this terrible little parasite has increased. We now know many similar to, but not identical with it, living in the blood of birds, reptiles, and frogs (see fig. 45).