"The two processes, islamisation and arabisation, were now at work together, but, although intimately interconnected, were by no means identical. Nor did they halt at the same frontiers. Islamisation, essentially a spiritual force, progressed much further aheld and was able to sweep barriers which arabisation, involving material displacement, could not always overstep. Broadly speaking, every country which became permanently arabised became also permanently islamised. But the converse is not true. There arc countries, such as Persia. and Afghanistan, where, notwithstanding a thorough and lasting islamisation, the progress of arabisation remained so restricted as to he, for our purposes, negligible."