But apart from the question of identity or diversity of small-pox and vaccinia, Pasteur's researches prove beyond all question that a disease virus may be both diminished and augmented in power by physiological devices, and that therefore the efficacy of the vaccine lymph may, in various ways, be so diminished as to lose its protective power, without shaking our faith in the principle of vaccination or detracting in the least from the inestimable value of Jenner's discovery. The attention of the scientific world will now be, and is, directed to the important inquiry, How far has the original vaccinia of Jenner lost its protective power? If so, how has this been brought about, and by what means can it be restored? Must we again revert to the cow for a new supply? Need we only be more scrupulous in the selection of the vesicles, and the particular stage of their development, and in the mode in which the operation of vaccination is performed? These and numerous other similar questions are now being discussed and investigated, but none probably is more important than the question how far the protective influence in each individual is dissipated by time, and hence the principle of re-vaccination is now being enforced. There can be no doubt that different epidemics possess different degrees of virulence, and what proves a sufficient protection in a mild epidemic of small-pox may not be sufficient in a more virulent one. In certain seasons and in certain conditions of the atmosphere, the human system is more prone to certain disease than at other times. Pasteur's experiments on cultivated virus or germs show that in the course of time, and in certain conditions of exposure to the action of oxygen or other agents, the vitality, or constitution, so to speak, of the germs may be so changed as materially to alter their action on the animal system. We have, therefore, scientific grounds for reverting from time to time to the heifer for a new stock, rather than continuing to rely on the perpetual transmission from one human body to another.