However, what seemed at the outstart as two incompatible conceptions of how languages change has coalesced today into one single explanatory theory. Hock already noted in 1991 (1991:454):[1]
“The discovery in the late nineteenth century that isoglosses can cut across well-established linguistic boundaries at first created considerable attention and controversy. And it became fashionable to oppose a wave theory to a tree theory... Today, however, it is quite evident that the phenomena referred to by these two terms are complementary aspects of linguistic change...
As demonstrated by Labov (2007)[2], what needed to be reconciled within one framework of thinking were the transmission and the diffusion principles of linguistic change. The transmission of change within a speech community is characterized by incrementation within a faithfully reproduced pattern characteristic of the tree model, while diffusion across communities shows weakening of the original pattern and a loss of structural features. This is the result of the differences between the learning abilities of children and adults as intercommunal contacts are primarily between the latter.
Notes
^ Hock, Hans Henrich. 1991. Principles of Historical Linguistics (2nd/rv/upd ed.). Berlin: Walter de Gruyter.
^ Labov, William. 2007. "Transmission and diffusion." Language 83.344-387.