In some ways it seems that the central idea here was a precursor of Niven's protectors or Wilson's "second adulthood", and at the end he introduces the beloved SF idea of eventual progression to bodiless energy beings. There's lots there that is queasy-making in a modern context, of course.
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http://en.wikipedia.org/wiki/Back_to_Methuselah
In some ways it seems that the central idea here was a precursor of Niven's protectors or Wilson's "second adulthood", and at the end he introduces the beloved SF idea of eventual progression to bodiless energy beings. There's lots there that is queasy-making in a modern context, of course.