Book II. deals with the constitution of the universe, including astronomical and meteorological phenomena, such as Meteors, Halos, Eclipses, Winds, Earthquakes, Rain, etc., etc. Many of these cover the same ground as the _Q.N._ Among the domestic authors cited for this Book are M. Varro, Livy, Cornelius Nepos, Caecina, “who wrote on the Etruscan cult”; among the foreign authors are Plato, Anaximander, Democritus, Archimedes, Aristotle, etc., etc. The omission of Seneca from the Latin list is balanced by that of Theophrastus from the Greek list. It is, of course, unsafe to build any theory on a merely negative basis. Obviously Pliny had read at any rate portions of these authors, to whom he elsewhere refers, and may, through mere oversight or negligence, have omitted specific mention of them here: he usually refers to authors and not to their individual works. If, at the time of the composition of Book II., which may have been considerably earlier than the date of publication of the whole work, he did not know of Seneca’s _Q.N._, then the inference seems inevitable that there were current a collection or collections of the opinions (δόξαι) of the older philosophers which were common property to any one interested in such matters. The _Placita_ attributed to Plutarch, though its present form may be much later than Pliny’s time, may have been derived from sources of this kind. We shall not be far wrong in supposing that, in addition to the works still extant, there was a mass of material available to Seneca and Pliny alike which represented the traditional views on physical and allied subjects handed down from the old Greek philosophy. Most of the Latin authors, seventeen in number in all, cited by Pliny on Book II. are now known to us only by name; of those whose works remain, Varro is the only one whom we should consider likely to furnish much material for the topic in hand. Of Pliny’s lists in general it may be said that they indicate that a good many writers even among the Romans had been attracted by subjects of a scientific or quasi-scientific character, if we may not venture to say that their works can rank as science even in the modified sense in which the term is applicable to Seneca or Pliny. It is in keeping with the character of the people that practical sciences like agriculture (Varro, Columella) and architecture (Vitruvius), not to mention cookery, should have received special attention. These authors, with others like Manilius (_Astronomica_) and Pomponius Mela (geography), however interesting in themselves, have only an indirect and sometimes only a remote bearing on the Physical Science of their day. VI. THE “QUAESTIONES NATURALES” IN THE MIDDLE AGES