A direct and practicable application of these data will be in the direction of explaining some of the pathological states in domestic animals and man. There are indeed many disease entities or syndromes in these groups for which no useful hypothesis has been advanced, while for others a partial explanation has been offered, usually, however, inadequate wherewith to form the basis of rational prevention or therapy. Thus, for example, essential emphysema seems to be limited to the animals of civilization; on the other hand, the anatomical basis of exophthalmic goitre may be seen throughout nearly the whole animal kingdom yet the clinical phenomena belong characteristically to man, and are occasionally seen atypically in the dog. While it may be impossible to give a complete comparative anatomy and physiology for each of the pathological states, the attempt will be made to treat all subjects analogically through the zoological orders. The experimental pathologist may find the records of the Garden useful in his work. For example, he can know that rodents are not prominent among the orders showing spontaneous arteriosclerosis, but that nephritis occurs among them in about a quarter of natural deaths; or he may learn that the Primates have a good cardiac reserve while the Marsupialia have not. Too often experimental work is not based upon facts including natural probabilities. A collection of pathological data such as is presented in the following pages may be of assistance to veterinarians and managers of zoological gardens in the diagnosis of sickness in animals, both wild and domesticated. We do not presume to offer a system of veterinary medicine, but it is possible to introduce certain objective findings of practical hygienic and therapeutic value. Such observations are, however, limited and in our experience at the Philadelphia Garden the diagnosis of disease in a wild animal, excepting of course those which are perfectly self-evident, is more often speculation and conjecture than at all well grounded. It is not uncommon for animals to come to autopsy presenting a perfect galaxy of abnormalities, yet the closest antemortem observation failed to reveal unusual conduct or appearance. On the other hand specimens are frequently opened whose organs fail to contain any lesions discoverable even by careful study. Dr. Henry Chapman, sometime prosector to the Society, once made a remark in this connection—“Why do they die or how can they live so long.” Space is given to this phase of the observation of wild animals in order to emphasize the difficulties of interpreting their conditions, but of course it should be understood that certain data of value may be gained by close attention to the details of their normal behavior and to changes which occur indicating that something is wrong.