Examples of such diseases are:—Scarlet Fever, Diphtheria, Small Pox, Cholera, Yellow Fever, Tuberculous Disease, Ague, Hydrophobia, Cattle Plague, Anthrax; and many others might be enumerated whose dependence upon parasitic Germs is almost conclusively proved. But these are sufficient to show the deep interest of this branch of study. Not only are these Germs found in the solids of the body in some of these diseases, but in others their presence is easily and constantly recognisable in the blood or other fluids, and even within the corpuscles which the blood so largely contains. And not only so, but the forms which these micro-organisms present are so constant and so definite in the different disorders, that it is now almost possible in some instances to diagnose what diseased condition we have to deal with by an examination of the fluid or tissue in which they are contained. It is, perhaps, but right here to say that some of our most cautious observers consider that it is as yet hardly _proven_ that the differing Germs found in the various diseases are their actual and efficient cause; but their definiteness in the various disorders, and the constancy of their presence, leave little doubt that this will hereafter be conclusively shown to be the case. Some of these Germs seem to have a short life and rapid development, and then we have acute disease. Others seem to have a more prolonged or continuing career, and then we have chronic disease. Whilst others seem to have an intermitting development, and then we have paroxysmal disease. Many curious facts have been observed in reference to the forms, or multiplication, or life-history of some of these parasitic beings. Thus, for example, in one class (_Filaria sanguinis_) it was noticed that the minute embryos of this little blood-worm could only be discovered in the evening or during the night, and not during the day; and the reason of this has appeared to be that it is necessary that they should come within reach of the Mosquito Gnat, which is a night-feeding animal, in whose bodies one stage of their development appears to take place. One of the latest discoveries in this department of natural history is that of the blood germs, which co-exist with the various forms of ague and malarious disease, and upon which they would appear to depend. Dr. Osler, of Pennsylvania (now Regius Professor of Medicine, at Oxford), has recently described and figured these little bodies in an elaborate paper full of interest.