It is of course universally recognised that Roman architects differed from their Greek or Hellenistic predecessors largely in the practice of spanning considerable spaces with arches, vaults or domes, whereas the Greeks limited themselves usually to stone architraves or wooden beams, with exceptional instances of the use of arches and probably simple trusses, of moderate dimensions. I am not certain, however, that it has occurred to investigators to discuss the possible cause for this radical difference between the two “classical” schools of architecture, except perhaps by generalisations as to racial qualities, merely another form of stating the same fact, rather than an explanation of it, or by reflections on the reckless waste of material and labour generated under a despotism, a form of explanation that will not carry conviction to the judicious. The matter is however of sufficient interest to justify the enquiry as to whether a more definite and convincing cause cannot be assigned.