The transmission of linguistic change within a speech community is characterized by incrementation within a faithfully reproduced pattern characteristic of the family tree model, while diffusion across communities shows weakening of the original pattern and a loss of structural features. It is proposed that this is the result of the difference between the learning abilities of children and adults. Evidence is drawn from two studies of geographic diffusion. (i) Structural constraints are lost in the diffusion of the New York City pattern of tensing short-a to four other communities: northern New Jersey, Albany, Cincinnati, and New Orleans. (ii) The spread of the Northern Cities Shift from Chicago to St. Louis is found to represent the borrowing of individual sound changes, rather than the diffusion of the structural pattern as a whole.