We evaluate whether John Holland's Echo model exemplifies his theory of complex adaptive systems. After reviewing Holland's theory of complex adaptive systems and describing his Echo model, we describe and explain the characteristic evolutionary behavior observed in a series of Echo model runs. We conclude that Echo lacks the diversity of hierarchically organized aggregates that typify complex adaptive systems, and we explore possible explanations for this failure. Keywords Echo, John Holland,

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... omplex adaptive systems, artificial life, genetic algorithms. c 2000 by the Massachusetts Institute of Technology Evolutionary Computation 8(4): 419-442 1 The source code and additional information on this version of Echo can be found at