While Crick {and his co-workers} cracked the genetic code-a set of instructions,...

While Crick {and his co-workers} cracked the genetic code-a set of instructions, in general terms, for building a body-Edelman realized early that the genetic code could not specify or control the fate of every single cell in the body, that cellular development, especially in the nervous system, was subject to all sorts of contingencies-nerve cells could die, could migrate {Edelman spoke of such migrants as "gypsies"}, could connect up with each other in unpredictable ways-so that even by the time of birth the fine neural circuitry is quite different even in the brains of identical twins; they are already different individuals who respond to experience in individual ways. Darwin, studying the morphology of barnacles a century before Crick or Edelman, observed that no two barnacles of the same species were ever exactly the same; biological populations consisted not of identical replicas but of different and distinct individuals. It was upon such a population of variants that natural selection could act, preserving some lineages for posterity, condemning others to extinction {Edelman liked to call natural selection "a huge death machine"}. Edelman conceived, almost from the start of his career, that processes analogous to natural selection might be crucial for individual organisms-especially higher animals-in the course of their lives, with life experiences serving to strengthen certain neuronal connections or constellations in the nervous system and to weaken or extinguish others.4 Edelman thought of the basic unit of selection and change as being not a single neuron but groups of fifty to a thousand interconnected neurons; thus he called his hypothesis the theory of neuronal group selection. He saw his own work as the completion of Darwin's task, adding selection at a cellular level within the life span of a single individual to that of natural selection over many generations. Clearly

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