The heart is an organ whose duty, throughout the two classes considered in this study, remains entirely identic, purely a physical one in driving the blood through the corresponding vascular system. The physics involved naturally differs between mammals and birds, but energy is derived from the automatic power lodged in the cardiac musculature. Whether this be neuromyogenic, as seems to be the case in all mammals, or purely myogenic, as is probably the case for the birds in which MacKenzie and Robertson[5] say there is no atrioventricular bundle, the result is the same, since in both classes there is some continuity of muscle fibres from auricle to ventricle. The gross anatomy varies little if any more than the physiology, albeit there is proportionately greater auricular capacity in the mammals than in the birds, and indeed there are differences within the classes which cannot now be readily explained; certain minor variations of valvular arrangement exist, such as the absence of the membranous light tricuspid in Aves. When, however, one considers the cardiac power available for various animals, the subject becomes one of greater breadth and complexity, for no consistency obtains even within families, since the demand for cardiac strength will vary more with habits than with zoological relationships. Thus for example the domestic rabbit has a small heart volume while the wild hare has a great one. Although, of course, the size of an organ may not be an absolute measure of its efficiency (a flea’s leg muscle has relatively greater power than a man’s), yet size is the only physical gauge one has for estimating nature’s preparation for expected demand. Perhaps this will be shown later when after discussing the pathological anatomy of the heart in the wild mammals and birds, we can study these changes in light of statistics upon the relative size of the heart. EXPRESSION OF CARDIAC DISEASE.