Primate vocal repertoires may be classified along a continuum of discrete vs. graded signals, with the placement of a particular repertoire a result of the interplay between the species'' social organization and adaptive specializations of individual signals engendered by the physical environment and their use. Discussion of the different types of graded signals reveals that they may possess either transitional, ontogenetic, or internal variability. Upon examination, the speech of humans is found to contain primarily internally graded signals, and it is suggested that the major phenomena of human speech perception are related to this internally graded quality. Based on this analysis of human speech, it is proposed that other species possessing internal gradings (e.g., Japanese macaques) are also likely to demonstrate similar perceptual phenomena for their own graded vocalizations.