just thought but if you take the analogy of fungi, they start from a centre and as they spread they can leave a centre bereft of fungi. Or take the phenomenon of rich and poor in cities. Rich folks living in the centre and as the city expands the rich want to be in "better" places so they migrate to the periphery, the centre is filled with the poor in the crumbling old buildings. But note, in a hundred years or so the centre becomes obscenely expensive again.

Now if "blues" had started in an urban environment like, say, New Orleans and spread from there, and fashion being what it is, "Jazz" came in to fill the foided centre...........................

we have that "delta" !

speculation can open eyes, but maybe not minds.