“The only criterion,” says Malthus, “of a real and permanent increase in the population of any country is the increase in the means of subsistence. But even this criterion is subject to slight variations, which, however, are completely open to observation. In some countries population seems to have been forced: that is the people have been habituated by degrees to live almost upon the smallest possible quantity of food. There must have been periods in such countries when population increased permanently without an increase in the means of subsistence. China, India, and the countries possessed by the Bedoween Arabs, as we have seen in the former part of this work, appear to answer to this description. The average produce of these countries seems to be but barely sufficient to support the lives of the inhabitants, and, of course, any deficiency from the badness of the seasons must be fatal. Nations in this state must necessarily be subject to famines.” Almost all the histories of epidemics which we have read tend to confirm the supposition that they are greatly caused by that over-population which, as in Dublin in 1880, leads to over-crowded houses filled by ill-fed and ill-clad inmates. Dr. Short, an author of the last century, shows in his work (Air, Seasons, &c., vol. ii. p. 206), that a very considerable proportion of the epidemic years either have followed or were accompanied by seasons of dearth and bad food. In other places he also mentions great plagues as diminishing particularly the numbers of the poorest classes; and in speaking of different diseases, he observes, that those which are occasioned by bad and unwholesome food generally last the longest. “We know (says our author) from constant experience that fevers are generated in our jails, our manufactories, our crowded workhouses, and in the narrow and close streets of our large towns, all which situations appear to be similar in their effects to squalid poverty, and we cannot doubt that causes of this kind, aggravated in degree, contributed to the production and prevalence of those great and wasting plagues formerly so common in Europe, but which now, from the mitigation of their causes, are everywhere considerably abated, and in many places appear to be completely extirpated.