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1. Experiments were designed to reveal geographic differentiation, and its evolutionary significance, in early life-history characters of Fundulus heteroclitus. The inherited capacity for growth was studied for five populations, from South Carolina to Maine, USA, at three temperatures representative of field conditions. Two generations of newly hatched fish were reared and tested. The second generation of fish was produced from parents that were themselves reared in the laboratory, in order to control for maternally transmitted environmental effects. 2. Over the first three weeks post-hatch, individuals from northern populations grew more rapidly than those from southern populations at higher temperatures (21 and 28 ^⚬C) whereas there were generally no differences among populations at the lowest temperature (17 ^⚬C). 3. The results do not conform to thermal adaptation models, in which northern populations are …