Wheeled vehicles. The zone defined by the first great wave of Islamic conquest was so strikingly homogeneous in transportation characteristics as to raise the suspicion that transportation played some role in defining the limits of expansion. The factors that dictated this homogeneity substantially antedated the life of Muhammad. Though some of the earliest evidence for the use of wheeled vehicles anywhere in the world goes back to Mesopotamia circa 3000 B.C.E., there were few wheeled vehicles in use in the regions the Arab armies invaded in the seventh century C.E. Evidence for this statement comes largely from the near absence of references to wheeled transport in medieval Islamic sources. Not only are they rarely mentioned, but Arabic and Persian are deficient in words relating to vehicles, and the infrequent miniature paintings depicting wheeled vehicles are usually so fantastic as to...