The difficulty of estimating the bison population of primitive America shrinks in comparison to trying to estimate the numbers in North America just after the Civil War — just before the start of the commercial hide hunt often called “the Great Slaughter.” Many forces — horseback hunting, robe trading, habitat degradation — bore down on bison in the eighteenth and nineteenth centuries. Even the weather turned against them. One may assume with reasonable certainty that the bison population west of the Mississippi River at the close of the Civil War numbered in the millions, probably in the tens of millions.