In the past few decades the techniques of physical acoustics have been applied to the study of auditory physiology with increasing sophistication. There now exists a body of data which can lead to a comprehensive theory of the function of middle ears as transmitters of acoustic signals. We have made many of these measurements on the cat, as an example of animals with a three-bone ossicular chain, and on the alligator lizard, as an example of animals with a one ossicle ear. The results are informative in that they are inconsistent with the widely held view that the middle ear functions as a mechanical transformer. With a few more selected measurements in cat and alligator lizard we should be able to improve our understanding of middle-ear acoustics to the point of generating mechanical circuit models in which the elements are relatable to anatemical entities. These results will suggest general relations between structure and function which we can test by taking advantage of the natural variations in the anatomy of the middle ear among different species. That is, measurements on several other species, which range over the vertebrate classes, will determine the extent to which the generalizations are correct. This work should be of great interest to acousticians, comparative physiologists, paleontologists, and communicatioons engineers. Also, there should be practical significance for autiologists and otologists in both the diagnosis and treatment of conductive hearing disorders.