It is one of the few consoling facts, well known to all students of comparative pathology or the diseases of the different species of animals, how peculiarly specialized they are in the choice of their diseases, or, perhaps, to put it more accurately, how particular and restricted disease-germs are in their choice of a host. For instance, out of twenty-eight actually infectious diseases which are most common among the domestic animals and man, other than tuberculosis, only one--_rabies_--is readily communicable to more than three species; only three--_anthrax_, _tetanus_, and _foot-and-mouth disease_--are communicable to two species; while the remainder are almost absolutely confined to one species, even though this be thrown into closest contact with half a dozen others. Again, we have half a dozen similar instances in the case of tuberculosis itself. The horse and the sheep, for instance, are both most intimately associated with cattle, pastured in the same fields, fed upon the same food, and yet tuberculosis is almost unknown in sheep and decidedly uncommon in horses, and when it does occur in them is from a human source. The goat is almost equally immune from both human and _bovine_ forms, while the cat and the dog, although developing the infection with a low degree of frequency, almost invariably trace that infection to a human source. There is, therefore, no _a priori_ reason whatever why we should be any more susceptible to bovine tuberculosis than the remainder of the domestic animals. It is only fair to say, however, that the animal whose diet--and appetite--most closely resembles ours, the hog, is quite fairly susceptible to bovine tuberculosis if fed upon the milk or meat of tuberculous cattle. Next came the particularly consoling fact that although nothing has been more striking than the great increase in the amounts of meat and milk consumed by the mass of the community during our last twenty years' progress in civilization, this has been accompanied not by any increase of tuberculosis, but by a _diminution of from thirty-five to forty-five per cent_. The allegation so frequently made that there has been an increase in the amount of infantile tuberculosis has been shown, upon careful investigation by Shennan of Edinburgh, Guthrie of London, Kossel in Germany, Comby in France, Bovaird in New York, and others, to be practically without foundation.