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Wilbur and Collins (1973) postulated that amphibians should adjust their developmental rates in response to environmental conditions. They suggested that this might be accomplished by the responses of the differentiation rate to the growth rate. We grew tadpoles in eight treatments, of which four initially had low growth rates and four had high growth rates. Three of the initially low treatments were switched to high growth rates at different points in the larval period. Three of the initially high treatments were simultaneously switched to low growth rates. All of the experimentally switched treatments showed responses of mass at metamorphosis consistent with the predictions of the Wilbur-Collins model. Five of the six switched treatments showed responses of larval period that were consistent with the predictions of the model and inconsistent with the predictions of a simpler model proposed by Travis (1984). The …