Theories of Media Change: Understanding New Media - Garnet Hertz

"Towards the end of his life, McLuhan and his son Eric embarked on a project to update the 1964 Understanding Media in response to his critics' requests to provide a solid basis for his drastic and metaphoric claims; the result was Laws of Media: The New Science , published by his son eight years after Marshall's death. Laws of Media defines a general theory of media change, constructing a tetrad model with four characteristics of media-in-transition: extension, obsolescence, reversal and retrieval."