THE EVOLUTION OF THE MAMMALIAN EAR

Abstract

Recent discussions of the mechanisms which have brought evolution 'about have led to a wide spread concurrence of opinion about the factors involved in the production of geographical races and the isolation of species, but they have not achieved the same general acceptance in their consideration of major evolutionary events, the coming into being of the various phyla or of such a class as the Mammalia, or of such great steps as the evolution of the craniate eye. Here a different process, with a different mechanism macro-evolution has been invoked by some workers. It is evident that we shall never know the steps which led up to the craniate eye, but we can expect to see and to understand much of the evolutionary history of the sound transmitting mechanism of the mammalian middle ear, one of the great steps for which an independent macroevolution is invoked. The problems presented by the evolution of the mammalian middle ear may be stated historically as follows: In 1601 A.D., J. Casserius showed that mammals possessed three bones in their middle ear, and birds, as represented by a goose, only one. By 1800, names had been given to these elements, and it had been recognized that in mammals the lower jaw was a single bone on each side which articulated with the squamosal bone of the skull, whilst in reptiles the lower jaw was made of several bones one of which, the articular, moved on a quadrate bone which was part of the skull. By 1820, Meckel showed that the mamnalian malleus first appeared as the hinder part of cartilage and corresponded with the reptilian articular, and lby 1818, C. G. Carus had suggested that the incus is the quadrate. In 1837, Reichert gave a connected account of the development of the visceral arches in mammals and birds, adopting Cartis' and Meckel's suggestions, and the whole story is now commonly known by his name. The great difficulty in envisaging interm-iediates between the modern reptile and the mammalian condition led to many alternative suggestions, so that the publication by E. Gaupp (1913) of an exhaustive treatise on the problem which established, by a consideration of all then available 'lines of evidence, the fact that the old reptilian jaw articulation was represented in mammals by that between the incus and malleus, was important. An admirable account of the whole matter will be found in E. S. Goodrich, Studies on the Structure and Development of Vertebrates, London 1930, chapter VIII.