failed to clear up the etiology of the disease. In the last-mentioned disease, there are excellent a priori reasons for believing that a living micro-organism of some kind is the essential etiological factor; but this hypothetical germ has eluded all researches. Possibly it belongs to an entirely different class of micro-organisms, as is the case with the blood parasite which is now recognized as the cause of the malarial fevers.
Having thus briefly reviewed the progress of our knowledge relating to the etiology of infectious diseases, I desire to call your attention to the question of acquired immunity from these diseases.
No questions in general biology are more interesting, or more important from a practical point of view, than those which relate to the susceptibility of certain animals to the pathogenic action of certain species of bacteria, and the immunity, natural or acquired, from such pathogenic action which is possessed by other animals. It has long been known that certain infectious diseases, now demonstrated to be of bacterial origin, prevail only or principally among animals of a single species. Thus, typhoid fever, cholera, and relapsing fever are diseases of man, and the lower animals do not suffer from them when they are prevailing as an epidemic. On the other hand, man has a natural immunity from many of the infectious diseases of the lower animals, and diseases of this class which prevail among animals are frequently limited to a single species. Again, several species, including man, may be susceptible to a disease, while other animals have a natural immunity from it. Thus, tuberculosis is common to man, to cattle, to apes, and to the small herbivorous animals, while the carnivora are, as a rule, immune; anthrax may be communicated by inoculation to man, to cattle, to sheep, to guinea-pigs, rabbits, and mice, but the rat, the dog, carnivorous animals, and birds are generally immune; glanders, which is essentially a disease of the equine genus, may be communicated to man, to the guinea-pig, and to field-mice, while house-mice, rabbits, cattle, and swine are to a great extent immune.
In addition to this general race immunity or susceptibility, we have individual differences in susceptibility or resistance to the action of pathogenic bacteria, which may be either natural or acquired. As a rule, young animals are more susceptible than older ones. Thus in man the young are especially susceptible to scarlet fever, whooping-cough, and other "children's diseases," and after forty years of age the susceptibility to tubercular infection is very much diminished. Among the lower animals it is a matter of common laboratory experience that the very young of a susceptible species may be infected when inoculated with an "attenuated culture" which older animals of the same species are able to resist.