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So many diverse plant species are poisonous to domestic livestock that it seems highly improbable that a universal competitive advantage is common to all poisonous species. Plant poisons may have originated in mutations that were of no direct adaptive value. Once these secondary chemical products became established in the physiologic systems of plants, their interactions with insects and rodents that consumed the seeds, seedlings, or herbage of the plant may have given them adaptive value. Evolution of resistance to poisons of plants by the herbivores probably has led to the proliferation of species in genera that contain many poisonous species such as Astragalus. The study of such relationships should be given priority in range ecology to achieve effective management of the range resource. Some poisonous plant species have also evolved allelopathic defense mechanisms that enhance competitive advantages. The coevolution of poisonous plants, large herbivores, and rumen microfloras offers intriguing possibilities for study that may answer questions basic to the future success of range management. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020