The present work is a continuation of my treatises on collateral subjects, and, like them, maintains the opinion, that great epidemics are epochs of development, wherein the mental energies of mankind are exerted in every direction. The history of the world bears indisputable testimony to this fact. The tendencies of the mind, the turn of thought of whole ages, have frequently depended on prevailing diseases; for nothing exercises a more potent influence over man, either in disposing him to calmness and submission, or in kindling in him the wildest passions, than the proximity of inevitable and universal danger. Often have infatuation and fanaticism, hatred and revenge, engendered by an overwhelming fear of death, spread fire and flames throughout the world. Famine and diseases, among which may be instanced the fiery plague of St. Anthony, were no less powerful in calling forth the chivalrous spirit of the crusades than the enthusiastic eloquence of Peter the Hermit—the Black Death brought thousands to the stake, and aroused the fearful penances of the Flagellants—while the oriental leprosy cast a gloomy shade over society throughout the whole course of the middle ages.
With all such commotions, the most striking events of the world are in intimate relation, and unquestionably, amid the changing forms of existence in the human race, more has always depended on the prevailing tone of thought than on the rude powers by which those events were produced. The historian, therefore, who would investigate the hidden influence of mind, cannot dispense with medical research. The facts themselves convince him of the organic union of the corporeal and the spiritual in all human affairs, and consequently of the innate vital connexion of all human knowledge. Hence, in a medical point of view, how vast is the field for observation presented by the history of popular diseases. Present bodily sufferings[348], are, collectively, but a step in the development,—but one phase of morbid life amid a long series of phenomena, and hence are not fully understood without a previous knowledge of the past, and historical research. How can we recognise the ring of Saturn as such, so long as our axis of vision is in its plane, and we see it only as a line. Great pestilences have vanished or been dispersed; from causes apparently the most insignificant, the most important consequences have resulted, and throughout the vicissitudes of danger and devastation, the operations of mighty laws of nature are everywhere manifested in the social tendencies of entire centuries.