The key to the understanding of cortex lies in the sensory pathways that relay in the dorsal thalamus. But the traditional concept of the sensory path must be revised in the light of new evidence that for each modality there are two or more parallel paths. The present inquiry is addressed to the question of the functional significance of parallel pathways, e.g., in the visual system, the tecto-pulvinar and the geniculostriate pathways. The striate cortex can be regarded as the core visual area and a core area, AI can also be identified in the auditory system. Are there general principles which describe relations between core and belt that apply to all mammals? Are the projections of the belt pathways more likely to overlap with each other? Do they project to the core cortex as well as belt? To answer these questions and to discover trends in the evolution of primates, three species will be studied, opossum, tree shrew, and bushbaby. Finally, the discovery of separate parallel pathways suggests that something of behavioral significance is being segregated. The question this proposal hopes to answer is: What?