damage in a kidney which has received injury sufficient to cause nephritis, but of course it does not explain the cause. In classification of nephritis different commentators have employed different standards according as they viewed the acuteness or chronicity of the process, or as the principal functionating structures, glomerulus, tubular epithelium, blood vessels, or supporting connective tissues, presented the most conspicuous changes. To these, clinicians have added phenomena of constitutional complication or of direct renal insufficiency. These latter being unavailable for us, we must fall back upon a classification based upon physical changes and to this end we have always used a slight modification of the Weigert method. This classification offers little in the direction of etiology except that toxins are believed to cause tubular changes, bacteria to produce glomerular lesions and vascular deficiencies to lie at the root of chronic interstitial nephritis. The origin of acute nephritis of chiefly degenerative character seems best explained by reference to some form of toxemia, whereas exudative processes, be they in glomerulus or supporting structures, seem to depend upon the direct action of bacteria. The origin of a chronic nephritis cannot be explained quite so readily. No one has answered with complete satisfaction whether a chronic process always begins with and proceeds from a single attack of acute disease, whether many acute attacks succeed upon one another or whether many small crops of agents successively attack the organ over a long time. Nor has an adequate explanation of the rôle of damaged blood vessels been given. It is reasonably easy in man to discover the existence of nephritis and of a possible cause; this is only true of acute cases in wild animals. Focal infections, those which might be the point of mobilization for bacteria sent to the kidneys, are frequently found in man but with exception of an occasional carious tooth, or a chronic osteitis are to be localized with difficulty in lower animals. In so far as the rôle of a single acute attack in the causation of chronic disease is concerned our material offers nothing, but some collateral or presumptive evidence may be mustered in regard to multiple infections.