We do not ignore the fact that small-pox, like other similar diseases having an epidemic character, may be absent for a length of time from certain districts and then break out again; nor that each epidemic has its period of increment and decrement, and varies in its degree of malignancy. But a full and careful review of the whole history of small-pox since the introduction of vaccination, proves to every unprejudiced mind that every recurring epidemic finds its victims, with comparatively few exceptions, among the unvaccinated, that its spread is arrested by renewed attention to vaccination and its vigorous enforcement, and that, even taking into account the countries and localities where from various causes it has been neglected, the mortality from this foul and fatal disease, small-pox, has been enormously reduced. Human lives have been saved, and human life prolonged to such an extent that it is impossible to estimate the benefits that mankind has derived from the genius and devoted patriotic labours of one man. That doubts and difficulties in connection with this subject, involving the well-being of the whole human race, have lately arisen, must be admitted. But there is good reason to believe that, by modern researches on the subject of epidemic diseases and the germ theory of disease, these doubts are already being dispelled, and that the difficulties will be speedily obviated. The grounds for this belief will be understood by the consideration of those scientific investigations to a brief detail of which we now proceed. The reader will then also be better able to judge of the propriety, and necessity of certain measures which, to the uninformed, must appear objectionable or even repulsive and arbitrary.