The direct analysis of the evolution of the brain from the data of fossil endocasts yielded a number of important structural principles, the most important of which appeared a few years after C. Darwin’s publication. The elaboration of other systems in the brain has been so much greater in the evolution of “progressive” mammals that their olfactory system has been overshadowed in size. The analysis of selection pressures that resulted in the evolution of neural control and of the nervous system is less common than the comparable analysis of behavior. According to the localization doctrine, neuropsychological functions are controlled or determined by localized structures in the nervous system. Peripheral auditory systems near the sense cells and in the medulla of living amphibians and reptiles have only a few hundred or perhaps a few thousand neurons. Encephalization is the major evolutionary principle that has been proposed for comparative neurology.