The reciprocal evolutionary relationships of butterflies and their food plants have been examined on the basis of an extensive survey of patterns of plant utilization and information on factors affecting food plant choice. The evolution of secondary plant substances and the stepwise evolutionary responses to these by phytophagous organisms have clearly been the dominant factors in the evolution of butterflies and other phytophagous groups. Furthermore, these secondary plant substances have probably been critical in the evolution of angiosperm subgroups and perhaps of the angiosperms themselves. The examination of broad patterns of coevolution permits several levels of predictions and shows promise as a route to the understanding of community evolution. Little information useful for the reconstruction of phylogenies is supplied. It is apparent that reciprocal selective responses have been greatly underrated as a factor in the origination of organic diversity. The paramount importance of plant-herbivore interactions in generating terrestrial diversity is suggested. For instance, viewed in this framework the rich diversity of tropical communities may be traced in large part to the hospitality of warm climates toward poikilothermal phytophagous insects. link to publication