Painstakingly excavated in the 1940s by Elton & Nicholson, the sales records of the Hudson's Bay Company remain a touchstone for ecological investigations.  Not only do they form one of the longest ecological data sets in existence, but they strikingly demonstrate the existence of a phenomenon long known among trappers.

The pelt harvest records clearly show the existence of "cyclic fluctuations" in the abundance of most furbearing species.  The cycles are clearest, however, for the lynx.