Johannes Schmidt accounted for this [special resemblances] by the so-called ' wave-hypothesis'. Different linguistic changes may spread, like waves, over a speech-area, and each change may be carried out over a part of the area that does not coincide with the part covered by an earlier change. The result of successive waves will be a network of isoglosses. Adjacent districts will resemble each other most; in whatever direction one travels, differences will increase with distance, as one crosses more and more isogloss-lines.
- Bloomfield (1935), Pag. 317