Analysis of the associated pathology will reveal that among the mammals, renal disease, chronic infections and diseases of the thoracic serosa are most often responsible for hypertrophy, and that something over one- third of the hearts showed myocardial damage. Among the Aves arteriosclerosis and renal disease are most important in enlarging the heart; half of the cases had myocarditis. In so far as dilatation in mammals is concerned, renal disease and acute infections are decidedly more important than other influences, even than the next in order— chronic infections and pulmonary diseases; only one-fifth of the cases had myocardial disease. Acute infectious disease is the most potent cause of dilatation in birds; only two of the eight cases had degeneration of the heart muscle. COMPARISON OF MAMMALIA AND AVES. If a comparison of the incidence of increased muscle bulk in the two classes be made[9] it will be found to occur two and one-half times more often in mammals, while dilatation occurs nearly ten times more often among the mammals than among the birds. Hypertrophy is accompanied by myocardial change in 44 per cent. of the cases, whereas muscular degeneration was only seen in 24 per cent. of the dilatations; this change is conspicuously lacking in the Primates, Ungulates and Marsupials. The usual teaching has been that dilatation, which means enlargement of chambers and thinning of walls or at least no thickening thereof, implied an inability on the part of the heart to keep up with increased demand—a decompensation. If Starling be correct that dilatation is not a degeneration of pump value but merely one of adaptations to increased demand, then this method is more characteristic of mammals than of birds. There is, however, the reserve power to increase the muscle bulk inherent in the mammalian, not possessed or needed by the avian heart. The large-hearted class Aves certainly dilate their blood pump less frequently than mammals and indeed have less cardiac disease. An analysis of the incidence of hypertrophy _versus_ dilatation shows that hypertrophying power resides in the Primates, Accipitres and Struthiones, their hearts relatively seldom dilatating. Lack of such power and consequent dilatation resides in Rodentia, Ungulata, Marsupialia and Anseres. Hypertrophying power lies therefore chiefly in the heart of average size for its class, dilatation occurring in the small heart. (See page 63.) AVIAN HYPERTROPHY.