"Between the eighth and tenth centuries Arabs brought back from India a variety of crops that they then began cultivating in the Middle East. These included staple crops such as hard wheat, rice, sugarcane, and new varieties of sorghum; fruits such as banana, sour orange, lemon, lime, mango, watermelon, and the coconut palm; vegetables such as spinach, artichoke, and eggplant; and the key industrial crop, cotton. From Iraq, many of these crops then spread westward all the way to Muslim Spain, which was transformed into a veritable garden under Muslim rule. Other crops passed by ship from southern Arabia to East Africa, while still others moved by caravan from northwest Africa across the Sahara to tropical West Africa. This was especially true for cotton, whose diffusion in Africa directly paralleled the spread of Islam itself."The passage can most directly be used to illustrate what aspect of the environmental history of the period 600-1450 C. E.?