Information published during the past century, especially the last decade, has identified pronounced species differences, not only in the morphological organization of ocular structures, but also in the functional responses of the eyes of different mammals to experimental and surgical procedures, as well as to drugs and autacoids. For the most part, these differences have been regarded as peculiarities or weakness rather than as fundamental evolutionary adaptations optimally suited to the environment and behavior of each species. This paper proposes a working hypothesis of evolutionary divergence in ocular defense mechanisms, based on some of the known morphological and functional differences among mammals, and discusses the implications of these differences with regard to the choice of appropriate animals for use as models in different areas of ophthalmic research.