If a particular phenomenon can arise very frequently and is highly stable once it occurs, it should be universal or near universal (...). If it tends to come into existence often and in various ways, but its stability is low, it should be found fairly often but distributed relatively evenly among genetic linguistic stocks (...). If a particular property rarely arises but is highly stable when it occurs, it should be fairly frequent on a global basis but be largely confined to a few linguistic stocks (...). If it occurs only rarely and is unstable when it occurs, it should be highly infrequent or non-existent and sporadic in its geographical and genetic distribution (...)
Greenberg (1978: 76)
An interesting quote (via Eitan Grossman; the exact source is presumably the book Universals of Human Language) that I ran into on a recent discussion session on a paper by Martin Haspelmath.
Joseph Greenberg gets a bad rep for his language classification overreaches, but if he has indeed already also formulated fundamentally important points such as this, I can now easily understand why he is still held in high esteem by modern typologists. As modern biologists say, nothing in biology makes sense without evolution; I hold that the same is true in linguistics with respect to language change.
