Summary(1. )The “ classical hypothesis ” which states that the heart of the frog functions in such a manner that the blood is distributed to various parts of the body in a selective manner is founded on no sound observational basis.(2. )Previous experimental work on the mode of action of the amphibian heart is contradictory.(3. )A new method of observing the mode of action of the heart is described.(4. )It is shown that in the frog the blood is not selectively distributed.(5. )The amphibian heart differs from those of other vertebrates in its methods of nutrition and respiration. With this is to be correlated the absence of a coronary circulation. These modifications are best explained as modifications to cutaneous respiration.(6. )In such structures as are available for palaeoutological study early amphibia differ very markedly from modern amphibia. Therefore it cannot be assumed that the type of heart possessed by ancient amphibia was necessarily similar to that of modern amphibia. Thus there is no reason to regard the amphibian heart, as exemplified by the frog, as being in any way part of an ascending evolutionary series standing between that of fishes and higher tetrapods.(7. )The experimental evidence of mixture of blood in the ventricle explains the viability of abnormal frogs in which the supposed “ classical ” form of selective distribution of the blood could no