Variation in _H. gracilis_, as shown in the graphic analysis (Figs. 1 and 2) and distribution map (Plate 21), presents two clines: a north-south cline and a large-river to small-river (mainly east-west) cline. The absence of _H. gracilis_ from certain portions of river systems is a matter of concern. The species has not been found in the lower Arkansas River and the Rio Grande, nor in sandy tributary creeks in eastern Kansas and Missouri that appear to provide suitable habitat. It has already been noted that _H. g. gulonella_ seems to be the ecological equivalent of _Semotilus atromaculatus_ in streams in which _S. atromaculatus_ is not found. _S. atromaculatus_ occurs in creeks of eastern Kansas and Missouri, and may provide interspecific competition that prevents establishment of the flathead chub in these creeks. Regardless of cause, the gaps in distribution of _H. gracilis_ tend to limit gene flow. Many characters used in the separation of the two subspecies are known to be influenced by environmental conditions, especially temperature. Hubbs (1922, 1926, 1941), Schultz (1927), Vladykov (1934), Taning (1952) and Weisel (1955), among others, have pointed out a correlation between temperature (or developmental rate of fish) and the number of vertebrae, scales, and fin-rays. Likewise, Martin (1949) and Hart (1952) have shown that the proportions of some body-parts vary in response to temperature during early development. In _H. gracilis_, the general nature of the clines found in a majority of characters (but not all characters) suggests a temperature influence. However, temperature-dependent variability that has so far been demonstrated experimentally in fishes is generally of lesser magnitude than the differences distinguishing _H. g. gracilis_ and _H. g. gulonella_. To our knowledge, the most extreme differences that have been induced by modification of temperature are those reported for _Salmo trutta_ by Taning (1952:181-182), who states: "Shock treatment produced by especially great changes in temperature (_c._ 10-14 deg. C), especially during the super-sensitive period [of somatic differentiation that fixes vertebral number] may produce ... a difference of 3-4 vertebrae ... in offspring of the same parents." The difference cited approximates that which distinguishes natural populations of _H. g. gracilis_ and _H. g. gulonella_. Although we cannot assume that the sensitivity of the brown trout is the same as that of the flathead chub, the causative conditions in Taning's study could scarcely be expected in nature; furthermore, it seems significant that extremely high (as well as extremely low) mean numbers of scales and vertebrae were found at southern localities, and that low mean numbers of scales and vertebrae were found as far north as Wyoming and Montana. We think it likely that temperature does influence the expression of characters in _H. gracilis_, directly in individual