This theory, too, is the well-known explanation of the protective power of the Cow Pox, which once having permeated a human system, has rendered it unsuitable for the future healthy and vigorous development of its greater relation—the Small Pox. In default of available means of destroying the Germs of other malignant diseases, prolonged efforts have been made (and notably by the great French pathologist, Pasteur) so to diminish the intensity of the destructive force of some of these specific Germs, that they may be safely inoculated into human bodies without danger to life, and yet be potent enough in their effects to anticipate and render abortive the invasion of the more virulent diseases. This has been attempted by repeated cultivations of the Germs in proper media, until after several of such generations the broods shall have acquired the requisite diminished vitality—in fact, until that diminution of virulence which the Small Pox Germ has sustained in passing through the Cow has been obtained by these artificial means. We are all familiar with the attempts which have recently been made by Pasteur in this direction, in regard to that most fatal disease Hydrophobia. It remains to be seen how far he has been successful in solving this preventive problem; and how far this may be the true method by which to utilize our knowledge of bacterial life. Medical men are diligently working at this subject from their own point of view. There is much to be done by microscopic naturalists in unravelling the life-history of these little beings; and we are glad to recognise the kind of results which may be hoped for in the future. Such considerations as these are fraught with matter for deep reflection, and tend to open our minds to the far-reaching possibilities not only of this special knowledge, but of that which we are gaining in many other branches of science. Each fresh item of knowledge is like a new step upon a ladder, and raises us to a fresh height from which we can take a wider survey, and which we can assume as a loftier and broader basis for further enquiry. Scientific thought is ever as to what may next be done, and how to do it. For, as Sir James Paget has recently said: “Every increase of knowledge brings before us a larger and clearer view of the immeasurable quantity which is still to be gained. The more we know, the more can we see, if we will, how much more there is that we do not know.” And of this we may be sure, that it is by minute and exact work only that in the future our store of knowledge is to be increased and made sure. If no other example to prove this were at hand, it would be sufficient to quote the recent observations of the Rev. Dr. Dallinger on the subject of the conjugation of the nuclei of some minute forms of cell life: observations which bring us nearer to some definite knowledge of this particular matter than any hitherto made upon higher classes of creatures.