clumping into complexes

Switching his vantage from earth to the clouds themselves John H. McWhorter tries to assess the boundaries (from The Power of Babel: a natural history of language):

“Typically, what looks from the air like ‘a language’ is actually a much hazier business on the ground. Korean is relatively uniform, but Japanese speakers in Tokyo can barely follow speakers from the Ryukyu Islands … Hebrew is pretty tidy, but its neighbor and relative Arabic differs so much from country to country that the various ‘dialects’ differ about as much as Spanish, Italian, and Portuguese do from one another. …

“The best we can say is that there are innumerable dialects in the world, related to each other to various degrees, sometimes clumping into complexes particularly close to one another, but generally not so close that all are mutually intelligible, with distances often so great between some of them that their speakers do not consider themselves to be speaking ‘the same thing’ in any sense.”