Now the Roman Catacombs are a vast gallery of ancient Christian art. Single specimens, indeed, may exist elsewhere, and wherever they are found they deserve attentive examination; but it is obvious that the largest and most valuable collection in the world is to be found in subterranean Rome; and here, if anywhere, the subject may be thoroughly sifted and settled. It is to the minuteness of the topographical researches of De Rossi that we are indebted for the solid basis on which the earliest chapters of the history of Christian art can now be made to rest. His plan here, as well as in every other branch of his subject, has been simply to note all he sees; and then, when he has collected sufficient materials, to arrange and generalise, not according to any preconceived theory of classification, but absolutely according to the strict, stern facts before him. If these facts, arranged in their true historical and geographical order, present a harmonious whole, and suggest or support a theory which can otherwise be shown to be probable, he accepts it gladly; if not, he is content to leave the facts to speak for themselves, and to trust to further discoveries, or to the ingenuity of future commentators, to introduce light and order where at present there may seem to be chaos. For he never shrinks from acknowledging his inability to explain this or that phenomenon, and he prefers to leave a matter in doubt rather than to dogmatise on insufficient authority. Such moderation naturally inspires confidence; and perhaps another presumption in favour of the impartial accuracy of his statements may be found in the fact that sometimes they militate against the theories of those who would assign to every monument a greater antiquity than it is entitled to, and sometimes against the theories of the opposite and more numerous school. On the whole, however, whatever his opinion may be as to the age of any particular monument that has been called in question, his testimony as to the free use of painting by the early Christians is distinct and positive, and supported by irrefragable proofs.