This work was supported by grants CCR-9204339 and BIR-9300800 from the United States National Science Foundation, a grant from the Digital Equipment Corporation, and by the Santa Fe Institute, Thinking Machines Corp., IBM, and Hughes Aircraft. This work was conducted while at: School of Life & Health Sciences, University of Delaware, Newark, Delaware, 19716, USA, ray@udel.edu; and Santa Fe Institute, 1660 Old Pecos Trail, Suite A, Santa Fe, New Mexico, 87501, USA, ray@santafe.edu.