What, then, are the circumstances of pellagra, and do these throw light upon the medieval prevalence of leprosy? Pellagra has been proved with the highest attainable scientific certainty to be due to a staple diet of bread or porridge made from damaged or spoilt maize. It followed the introduction of maize into Lombardy at an interval of two or three generations, and its distribution corresponds closely to the poorer kinds of maize on colder soils, and to the class of the peasantry who get the worst kind of corn or meal for their food. The cases of the disease among the peasantry of Lombardy and some other maize-growing provinces of Northern Italy, were about one hundred thousand when last estimated; the endowed charitable houses and lunatic asylums are full of them. The connexion of the disease with its causes is perfectly well understood; but the economic questions of starvation wages, of truck, of large farms with bailiffs, and of agricultural usage, have proved too much for the chambers of commerce and the Government; so that there is as yet little or no sign of the decline of pellagra in the richest provinces of Italy. This disease is not mentioned in the Bible, therefore it has no traditional vogue; it is not well suited to knight-errantry, because it is a common evil of whole provinces; its causes are economic and social, therefore there is no ready favour to be earned by systematic attempts to deal with them; and there is absolutely no opening for heroism and self-sacrifice of the more ostentatious kind. These are among the reasons why this great object-lesson of a chronic disorder of nutrition, proceeding steadily before our eyes, has been so little perceived. It is in pellagra, however, that we find the key to the ancient problem of leprosy. The two diseases are closely allied in the insidious approach of their symptoms, in their implicating the tissue-nutrition through the nerves, or the nervous functions through the nutrition, in their cumulating and incurable character, and in their transmissibility by inheritance. Thus nosologically allied, they may be reasonably suspected of having analogous causes; and as we know the cause of modern pellagra to be something noxious in the habitual diet of the people, we may look for the cause of medieval leprosy in something of the same kind. The dietetic cause is not far to seek, and it cannot be stated better than in the following well-known passage by the philosophical Gilbert White in his _Natural History of Selborne_[206]:--