It might be contended that observations upon diseased states in captive animals would not represent natural developments, in other words, not that which occurs in the wild. Such indeed may be true in regard to the infectious diseases, but since we are imperfectly informed as to the pathology of the wild state, we are obliged to accept and use the best substitute at hand. Moreover it seems perfectly fair to consider as characteristic for an animal or group, the physical and even physiological expressions of morbid agencies as we know them, even though the animals be at the time under conditions not natural to them. It would be perhaps incorrect to say that cirrhosis of the liver occurs in .6 per cent. of animals in the wild as is the case for our autopsies, since incorrect food and infections are potent in its causation; on the other hand, our experience and some few data from naturalists and pathologists make it conceivable that tumors occur to this number in native states. The incidence of tumors in wild rodents is quite well known. Degenerations and fibroses, the result of parasitism, are known to exist throughout the entire animal kingdom. Further to illustrate how pathology is distributed in wild life, Plimmer’s experience[2] with 500 rats (_M. decumanus_) might be cited. He found the following: Tubercle 3 times, tape worm cysts 10, Tryp. lewisi 49, empyema 2, tumor of jaw from old injury 1, pleuritis and hydrothorax 1. Bacteria were found in 71 rats, 40 times in the lungs, 31 times in the spleen; saccharomyces were found 16 times in the lungs. Dr. W. L. Abbott reports to us personally that he has repeatedly found coiled exproventricular worms in the wild specimens he has collected. Not only are we informed of some isolated and individual pathological states but the existence of epizoötics of communicable disease among wild life is well authenticated. The simple citation of the extermination of deer in one section of Colorado by pleuropneumonia will suffice to illustrate this point. Other examples are, however, interesting. The occurrence of changes in the jaw bone almost certainly those of actinomycosis is reported by Blair, the specimens being shot in the wild and believed never to have been near civilization. The white-tailed deer of the Swan River Valley in Montana, are known to be constant carriers of liver-flukes. It would seem therefore that it is not unfair to use material gathered from animals under somewhat unnatural conditions as representing the reaction of the zoological orders to pathogenic agencies. Such conclusions must however be made very guardedly, for it is probable that not over ten per cent. of the total number of mammalian and avian species are to be observed in captivity. Because of the number of orders and the great variety of genera included in the present study it is probable nevertheless that the lesions are fairly representative of the whole animal kingdom.