[Illustration: FIG. 6. Growth in a group of frogs, each marked while still short of adult size and mostly recaptured after lapse of one or more hibernation periods. Each line connects records of an individual frog.] Differences in size between species and geographic variation in size in _Gastrophryne_ have been given little attention by herpetologists, but if understood, would help to clarify relationships. Hecht and Matalas stated in their revision (1946: 5) that size is of no importance as a taxonomic character, as typical _carolinensis_, _olivacea_, and _mazatlanensis_ all averaged approximately the same--26 to 28 mm.--females slightly larger than males. However, they arbitrarily classed as adults all individuals 22.5 mm. in length or larger, having found individuals this small that showed the darkened and distensible throat pouches characteristic of adult males. From the trend of my own measurements of _G. olivacea_ in northeastern Kansas, I conclude that either many immature individuals were included in their samples, or that the populations sampled included some with individuals that were remarkably small as adults. [Illustration: FIG. 7. Growth in another group of frogs that were marked as young or small adults and recaptured after intervals of more than a year. Frogs of this group were, on the average, larger than the individuals shown in Fig. 6, and they made less rapid growth.] The population which I studied may be considered typical of _G. olivacea_. They averaged large, including individuals up to 42 mm. in length, well above the maximum sizes for any reported in the literature. At metamorphosis these _olivacea_ are of approximately 50 percent greater length than _G. carolinensis_ as reported by Wright and Wright (1949: 573) and Anderson (1954: 41). Yet Blair (1950: 152) observed that in eastern Oklahoma, where the ranges of _olivacea_ and _carolinensis_ overlap, the latter is larger. On the basis of field and laboratory observations he tentatively concluded that one of the main barriers to interbreeding was the reluctance of the males of _carolinensis_ to clasp the smaller females of _olivacea_. That size differs in different populations, and is still poorly understood, is illustrated by the following discrepant figures from various authors. TABLE 4. SIZE RANGE OF ADULTS IN VARIOUS POPULATIONS OF GASTROPHRYNE.