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In this volume Guggenheim brings together the evidence for his regression theory of otosclerosis. This theory is based on four main points: 1. The bilateral symmetrical involvement in otosclerosis speaks for heredity. 2. The areas commonly involved are in intimate relationship to the oval window, round window and cochlea, structures which are recently acquired compared to the utriculus and canals and which are, therefore, more subject to variation. When these structures are involved in otosclerosis the result is strikingly similar to the morphologic features of the lowest vertebrates. Elaborating on this point is an interesting chapter on phylogenesis, tracing the development of the aural capsule from the lowest vertebrates, the cyclostomata (hagfish and lamprey) through the true fish, amphibia, reptiles and birds to mammals. 3. The area of predilection anterior to the oval window near the fissula ante fenestram is almost constantly involved in otosclerosis. The fissula is a