A reductionist approach to allometric analysis is proposed: (1) growing two or more closely related taxa in a common environment, (2) measuring changes in morphological dimensions over measured time intervals, and (3) calculating and comparing specific growth rates of parts. This approach complements studies of allometry which use field-collected specimens and bi-variate or multivariate analyses of logarithms of morphological distance measures. While field, cross-sectional methods are more robust and easily applied, longitudinal studies provide a new class of information-absolute growth rates of parts. This approach is illustrated by comparing two geographic populations of the Hermit crab Pagurus longicarpus. Ontogenetic equivalence is approximated by use of a reference dimension which exhibits the same initial sizes and specific growth rates in both populations. Differences between crabs from Connecticut and North Carolina in growth of the right claw relative to the thorax and in right/left claw symmetry are shown to be reducible to absolute differences in right claw growth. Further, geographic comparisons at separate ontogenetic stages show that both absolute and relative measures of growth change during the ontogeny, and geographic differences apparent in mid-ontogeny disappear in late ontogeny. Functional constraints on the growth of the right claw provide a probable explanation for both the ontogenetic and the geographic patterns.