development, and indirectly as a selective mechanism in the evolutionary process. The extent to which each kind of influence exists can be proved only by experimental work with both subspecies, which we hope to undertake at a later date. Other environmental factors that may have selective influence in this species are rate of current, volume of flow, and turbidity. Interaction of these environmental factors could result in genetic fixation of morphological characters through natural selection. The characters that distinguish _H. g. gracilis_ from _H. g. gulonella_ seem adaptive to life in large rivers and small streams. Evidence that these characters are under limited, direct environmental influence is found among populations in the Arkansas River System. Although populations in the Arkansas River have no continuity with populations of _H. g. gracilis_, upstream-downstream variations like those found in other river systems are apparent, but in lesser degree. The direction of variation in the Arkansas River is the reverse of that in the Platte and other tributaries of the Missouri River. For example, the populations farthest upstream (Florence, Pueblo) have slightly higher mean numbers of lateral line scales than do populations from Kansas, downstream. A remarkable effect of extreme parasitism in _H. gracilis_ has been described by Hubbs (1927). Very young chubs that harbored numerous tapeworms (_Proteocephalus_) had unusually large numbers of lateral-line scales, large eyes, short snouts, small fins, small mouths lacking barbels, and coalescent nares (internarial bridge weak or absent). Some of these abnormalities presumably resulted from retention of larval characteristics of the fish, correlated with the degree of infestation by tapeworms. No teratological adults were found, indicating that severe infections prevent survival to maturity. _H. g. gracilis_ occurs in three separate river systems (Mackenzie, Saskatchewan, Missouri-Mississippi) from latitude 36 deg. N to 66 deg. N, and longitude 89 deg. W to 123 deg. W. _H. g. gulonella_ exists as several seemingly-isolated populations in the upper parts of the Rio Grande, Pecos, South Canadian, Cimarron, Arkansas, Platte, and upper Missouri basins, from latitude 35 deg. N to 48 deg. N, and longitude 97 deg. W to 100 deg. W.