Such facts as these, when regarded merely from the point of view of a member of the Medical Profession have their deep and _special_ significance. But I am here to-night as a member of the Naturalists’ Society, and by all of us, in that capacity, they are first to be regarded, not as illustrations of disease, but of the life history of some of nature’s creations—creations which are no doubt as important, as definite, and which play as large a part in the general scheme of life, as do many of the larger forms of animated beings. Some of these little bodies, especially those of the bacillary and bacterial class, are extremely minute, and are best examined with powers ranging from one-tenth to one-twenty-fifth of an inch; and it is impossible to ignore the fact that their study requires not only good instruments, but much patience and skilled attention. Nevertheless, such study fairly comes within the scope of this Society’s work, and will most certainly repay any of its members who may be induced adequately to undertake it. Nor let it be said that minuteness is any reason for lack of interest on the part of the naturalist. For many of these micro-organisms have already been shown to be as varied, and to have as definite a structure, and as special a life history, as any of the larger types of beings; whilst, of course, we all recognise that apparent size is as nothing, that it is a mere accident, a question of the construction of our enquiring eyes, a condition that is at once altered and rectified by a magnifying glass. If it be true that the invasion and presence of various small organisms in the blood or tissues is the cause of the various specific diseases to which I have alluded, then the application of such knowledge as naturalists can obtain, as to the food and other conditions necessary to their existence; their mode of ingress to the body; their development and multiplication, becomes clear and obvious. It opens up to our minds possibilities both of prevention, and of either mitigation or cure. For it is evident that if we can starve these Germs of their necessary nutriment, or make their new habitation unsuitable for their healthy and vigorous development, their career as invaders will necessarily either be cut short, or be rendered feeble and impotent; and therefore the disease-changes which they can produce less violent and less lethal. Something of this kind appears naturally to have taken place in those persons in whom some of the zymotic diseases (of which Measles, Scarlet Fever, Whooping Cough, etc., may be taken as familiar types) have once run their course; in those, that is, who are popularly said to have already had these diseases. And although the exact abiding change which is produced has not been ascertained, yet it is well known, and quite understood, to be one which renders the fluids or tissues partially or wholly unsuitable for their future healthy growth.