Whereas Ivan P. Pavlov (1849-1936) is well-known for his work on classical conditioning, his contribution to neuroscience, particularly his interest in the function of neural centers in the central nervous system, is not as widely known. During the last three decades of his life, Pavlov explored cortical processes by salivary reflex conditioning, a method he used to develop his theory of higher nervous activity. This theory outlined the function of the brain in higher organisms in their interaction with the changing environmental contingencies. As early as 1908, Pavlov outlined a neurophysiological theory as the physiological basis of his theory of higher nervous activity. He maintained that the neural processes of excitation and inhibition irradiate and concentrate among the cortical neural centers. Most of all, he emphasized the plasticity of the cortex in higher organisms' in the Darwinian struggle for existence.