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This article presents information on the physiology of brain evolution. Recent research by Physician Paul D. MacLean, chief, Laboratory of Brain Evolution and Behavior, National Institute of Mental Health, Bethesda, Maryland, focused attention on an evolutionary gap, a generation gap, that applies to the brain and behavior. Learning to recognize, to understand and to live with this critical intra-brain gap and its effects on behavior may be more crucial than anything else to man's survival. To understand the brain's developmental gap, it is necessary to think in evolutionary terms. In the age of reptiles, 250 million years ago, non talking animals began developing components of the human brain. The primate forebrain is characterized by three basic patterns known as reptilian, paleomammalian and neomammalian--all of which are radically different in structure and chemistry, and which, in an evolutionary sense, are countless generations apart. In mammals the major counterpart of the reptilian forebrain includes a group of large ganglia called the R-complex.