  The purpose of this paper is to abstractly describe the notion of a
generative mechanism that implements a code and to provide a number of examples
including the DNA-RNA machinery that implements the genetic code, Chomsky's
Principles & Parameters model of a child acquiring a specific grammar given
`chunks' of linguistic experience (which play the role of the received code),
and embryonic development where positional information in the developing embryo
plays the role of the received code. A generative mechanism is distinguished
from a selectionist mechanism that has heretofore played an important role in
biological modeling (e.g., Darwinian evolution and the immune system).
