Then assume that at the end of the third generation, fertility falls to replacement (for simplicity assume that to be two). Now take the population forward in time to the next generation, line two of the spreadsheet. The first generation dies, and the new generation, the fourth, is equal to the third (because now fertility is replacement). Repeat the process again to reach the fifth generation (line 3 in the spreadsheet). The fifth generation is again equal to the fourth and now the population’s three generations are equal, and the population has reached equilibrium.