The Black Death, which here concerns us immediately, is one of the two great instances of a plague-virus with vast diffusive power, enormous momentum, and centuries of endurance. So great effects may be said to postulate adequate causes; and one must assume that the virus had been bred from cadaveric decomposition in circumstances of peculiar aggravation and on some vast or national scale. The sequence of events carries us to China; and the annals of China do furnish evidence that the assumed cause was there on a vast scale through a long period of national disaster, while the national customs of China for the disposal of the dead, like those of ancient Egypt, point to the existence of a real risk from allowing the soil to be permeated at large by the crude or hasty products of cadaveric decomposition. It is our duty to construct the best hypothesis we can, sparing no labour. No one really dispenses with theory, whatever his protestations to the contrary; those who are the loudest professors of suspended judgment are the most likely to fall victims to some empty verbalism which hangs loose at both ends, some ill-considered piece of argument which ignores the historical antecedents and stops short of the concrete conclusions. It has been so in the case of infective diseases, and of bubo-plague in particular. The virus of the plague, we are told, is specific; it has existed from an unknown antiquity, and has come down in an unbroken succession; we can no more discover how it arose, than we can tell how the first man arose, or the first mollusc, or the first moss or lichen; its species is, indeed, of the nature of the lowest vegetable organisms. The objection to that hypothesis of plague is that it involves a total disregard of facts. It is a mere formula, which saves all trouble, dispenses with all historical inquiry, and appears to be adapted equally to popular apprehension and to academic ease. The bubo-plagues of history have not, in fact, been all of the same descent; notably the Black Death was a wave of pestilence which Mohammedan countries, accustomed as they had been to native bubo-plagues for centuries before, recognized as an invasion from a foreign source, as an interruption of the sequence of their own plagues. Again, the attempt to link in one series the various scattered and circumscribed spots of plague now or lately existing must fail disastrously the moment it is seriously attempted. The hypothesis of one single source of the plague, of a species of disease arising we know not how, beginning we know not when or where, but at all events reproduced by ordinary generation in an unbroken series of cases, _ab aevo, ab ovo_, is the merest verbalism, wanting in reality or concreteness, and dictated by the curious illusion that a species of disease, because it reproduces itself after its kind, must resemble in other respects a species of living things.