But if we extend our inquiries to the diseases of nations, and of the whole human race, science is mute; as if it were not her province to take cognizance of them, and shows us only an immeasurable and unexplored country, which many suppose to be merely a barren desert, because no one to whose voice they are wont to listen, gives any information respecting it. Small is the number of those who have traversed it; often have they arrested their steps, filled with admiration at striking phenomena; have beheld inexhaustible mines waiting only for the hand of the labourer, and, from contemplating the development of collective organic life, which science nowhere else displays to them on so magnificent a scale, have experienced all the sacred joy of the naturalist to whom a higher source of knowledge has been opened. Yet could they not make themselves heard in the noisy tumult of the markets, and still less answer the innumerable questions directed to them by many, as from one mouth, not indeed to inquire after the truth, but to obtain a confirmation of an anciently received opinion, which originated in the fifth century before our era. Hence it is, that the doctrine of epidemics, surrounded by the other flourishing branches of medicine, remains alone unfruitful—we might almost say stunted in its growth. For, to the weighty opinions of Hippocrates, to the doctrines of Fracastoro which contain the experience of the much-tried Middle Ages, and lastly to the observations of Sydenham, only trifling and isolated facts have been added. Beyond these facts there exist, even up to the present times, only assumptions, which might, long since, have been reduced to their original nothingness, had that serious spirit of inquiry prevailed which comprehends space and penetrates ages.