For most of the agrarian era, the world was divided into four separate and distinct world zones. Over time, these zones slowly became more connected as networks of communication and exchange expanded. While innovations did occur throughout this era, such as irrigation, iron plows, and fast-ripening rice, none of these innovations were able to sustain long-term population growth, which limited expansion. Each innovation led to immediate growth, but once populations had grown beyond a certain point, they fell. These cycles of rise and fall in population, called Malthusian cycles, characterized the agrarian era. Humans would not break out of these cycles until the world zones became more connected and rates of innovation were capable of sustaining growth over much longer periods of time.