The immense and complicated structure of modern commerce has been erected on a foundation of material and social conditions, part of which is clearly visible to the least observant, whilst the other part is less obvious, though not less essential. In the social basis of modern commerce, we find something entirely new, and it is, therefore, in the direction that we must look for an adequate explanation of its continuous and immense expansion. The reign of Justinian affords the most natural halting-place in our rapid survey of the commerce of the Dark Ages. Desert Arabia was a barrier or a bridge between two great centres of civilization and commerce. The main hindrances to the development of commerce, in Asia as in Africa—the extreme irregularity of the caravans and the heavy exactions to which they were subject—have already been indicated.