This is one of the best books I've read about cultural evolution in many years. It focuses, obviously, on "traditions", defined (p. 37) as "anything that is widely distributed in a population", with the distribution being "due to a diffusion process".
It says how long ago something was said, so you can tell last month
from 2014 without doing the arithmetic. Full and green is recent; it
drains and cools as the date gets older, and empties completely at about
6 years.
5d5 days ago
8mo8 months ago
2yr2 years ago
4yr4 years ago
9yr9 years ago
A striped bar means the date itself is an estimate — the source did
not carry an exact one. An entry with no date at all gets no bar.