Most developments of civilization demand a settled system of agriculture, and are not compatible with nomadic life. The exact connexion between the rise of civilization and the rise of agriculture is possibly uncertain. The cultivation of different plants developed in different places: rice in the East and wheat in the West, for example. The cultivators may have attained only a low level of culture, and thousands of years may have elapsed before they produced a civilization of which the marks have endured. But to know where the different species of cultivated plants originated must help to trace the origins and diffusion of civilizations.