Strabo  says that Roman trade in the Indies was much more considerable than that of the kings of Egypt, and it is singular that the Romans, who had little familiarity with commerce, were more attuned to trade with the Indies than the kings of Egypt had been, who had it, in a manner of speaking, before their eyes. This requires an explanation.
After the death of Alexander, the kings of Egypt established a maritime trade in the Indies, and the kings of Syria, who had the easternmost provinces of the empire, and consequently the Indies, maintained this trade, of which we have spoken in chapter vi, which was carried on by land and by rivers, and which had been further facilitated by the establishment of the Macedonian colonies ; and so it was that Europe communicated with the Indies both through Egypt and through the kingdom of Syria. The dismemberment which was done of the kingdom of Syria, whence was formed that of Bactriana, had no harmful effect on that commerce. Marinus the Tyrian, cited by Ptolemy,  speaks of the discoveries made in the Indies by means of some Macedonian merchants. Those which the expeditions of kings had not made, the merchants made. We see in Ptolemy  that they went from the Stone Tower  to Sera ; and the merchants’ discovery of such a remote outpost, situated in the eastern and northern part of China, was a sort of marvel. Thus, under the kings of Syria and Bactriana, commodities from the south of India passed by the Indus, the Oxus, and the Caspian Sea to the west ; and those of the more easterly and northerly regions were carried from Sera, the Stone Tower, and other intermediate ports to the Euphrates. These merchants made their way by holding more or less the fortieth degree north latitude, through countries which are to the west of China, better constituted than they are today, because the Tartars had not yet infested them.
While the Syrian empire was extending its trade so far on land, Egypt was not greatly increasing its maritime trade.
The Parthians appeared, and founded their empire ; and when Egypt fell under the might of the Romans, that empire was in its prime and had received its extension.
The Romans and the Parthians were two rival powers which fought not to see which would reign, but which would exist. Between the two empires deserts were formed ; between the two empires, they were always under arms ; far from there being any trade, there was not even any communication. Ambition, jealousy, religion, enmity, and ethos separated everything. Thus commerce between Occident and Orient, after having several routes, was left with just one ; and Alexandria having become the only port along the route, that port grew.