either or whether?

Just as we have English, Chinese and Arabic languages, we have jazz, Blackfoot Indian and Tibetan chant musics. Like languages, musics have stylistic, geographical and sociological boundaries. They can be grouped into families; patterns of *descent, blending and development can be traced. Such diversity and patterning in both languages and musics arises from the processes of cultural transmission from one generation to the next and from one society to another. This can make identifying the boundaries between languages and musics difficult, ( ) in a historical sense (when did Old English become Middle English, or Classical music become Romantic?) or in the contemporary world (where, for instance, are the boundaries between folk, blues, gospel, country and jazz?).

In the blank, which should be filled in, 'either' or 'whether'?
And why so?