The rodents have been generally regarded as basically herbivorous animals. A review of the literature of the natural history of the feeding habits of rodents shows many species in all major lines of rodent evolution to be, to a surprising degree, carnivorous, piscivorous, or insectivorous. It is argued, therefore, that the primitive adaptation of the rodent mandibulo-dental apparatus was for an omnivorous diet rather than a herbivorous one, and that, indeed, the versatility of the feeding adaptation was the primary factor in the highly successful adaptive radiation in the order. A functional-morphological study of the rodent gnawing adaptation supports the argument.