A great proportion of the maladies to which mankind are liable have, it is true, remained entirely unaltered in their character and consequences from the earliest periods of medical history down to the present day. Synocha, Gout, and Epilepsy, for instance, show the same symptoms and course now, as the writings of Hippocrates describe them to have presented to him upwards of two thousand years ago. The _generatio de novo_ of a really new species of disease “is (says Dr. Mason Good[2]) perhaps as much a phenomenon as a really new species of plant or of animal.” Dr. Good’s remark is probably too sweeping in its principle; for, if necessary, it might be easy to show that, if the particular diseases of particular animal species are liable to alteration at all, they must necessarily alter more frequently than those animal species themselves. In pursuing such an inquiry, the pathologist labours under comparative disadvantages. The physiologist can, by the aid of geological research, prove that the individual species of plants and animals inhabiting this and other regions of the earth, have again and again been changed. The pathologist has no such demonstrative data to show that, in the course of time, the forms and species of morbid action have undergone great mutations, like the forms and species of normal life. But still we have strong grounds for believing that, in regard to our own individual species alone, the diseases to which mankind are subject have already undergone, in some respects, marked changes within the historic era of medicine. Since the first medical observations that are now extant on disease were made and recorded in Greece, various new species of human maladies have, there can be little doubt, made their original appearance. I need only allude to small-pox, measles, and hooping-cough. Again, some diseases which prevailed formerly, seem to have now entirely disappeared from among the human race—as, for example, the Lycanthropia of the Sacred Writings, and of Oribasius, Aetius, Marcellus, and various old medical authors.[3] Other maladies, as that most anomalous affection, the English sweating-sickness of the fifteenth century, have only once, and that for a very short period, been permitted to commit their ravages upon mankind. And lastly, we have still another and more extensive class, including maladies that have changed their geographical stations to such an extent, as to have made inroads upon whole districts and regions of the world, where they were formerly unknown, leaving now untouched the localities which, in older times, suffered most severely from their visitations.