It is one thing to admit the ingenuity, or even the plausibility, of a writer’s views, and another to accept them as articles of belief. So far from claiming for O’Brien that he has completely solved the mystery of the Round Towers, we may even confess a doubt that the latter admits of any complete solution. Certain links in the chain of evidence are wanting, which, to all appearance, are not likely to be ever supplied. That, for instance, the _Tuath-de-danaans_ came from Persia, bringing with them to Ireland their arts and their religion, is quite possible; but the absence of any reference to such migration in the more ancient Persian historians, where we should expect to find it; the want of some adequate explanation of the motives which could have led a highly-civilised people, accustomed to a luxurious climate, to prefer as their final settlement the bleak shores of a remote Atlantic island to the more temperate and, to an Eastern eye, more beautiful countries through which they must have passed on their way; the all but complete failure to point out the route which they followed in their quest of an asylum--these are gaps which require to be filled up before most of us will be prepared to accept their Eastern genealogy. Still, it must be confessed that O’Brien’s theory rests upon other and surer foundations, so far as its essential probability is concerned; also, that it is entertaining and suggestive to a degree which renders it, if not a profitable, at least a pleasing mental exercise.