The _Natural History_ has been criticized as poorly arranged and lacking in scientific classification, but this is a criticism which can be made of many works of the classical period. Their presentation is apt to be rambling and discursive rather than logical and systematic. Even Aristotle’s _History of Animals_ is described by Lewes[140] as unclassified in its arrangement and careless in its selection of material. I have often thought that the scholastic centuries did mankind at least one service, that of teaching lecturers and writers how to arrange their material. Pliny seems rather in advance of his times in supplying full tables of contents for the busy emperor’s convenience. Valerius Soranus seems to have been the only previous Roman writer to do this. One indication of haste in composition and failure to sift and compare his material is the fact that Pliny sometimes makes or includes contradictory statements, probably taken from different authorities. On the other hand, he not infrequently alludes to previous passages in his own work, thus showing that he has his material fairly well in hand. [Sidenote: His scepticism and credulity.] Pliny once said that there was no book so bad but what some good might be got from it,[141] and to the modern reader he seems almost incredibly credulous and indiscriminate in his selection of material, and to lack any standard of judgment between the true and the false. Yet he often assumes an air of scepticism and censures others sharply for their credulity or exaggeration. “’Tis strange,” he remarks _à propos_ of some tales of men transformed into wolves for nine or ten years, “how far Greek credulity has gone. No lie is so impudent that it lacks a voucher.”[142] Once he expresses his determination to include only those points on which his authorities are in agreement.[143] [Sidenote: A guide to ancient science.]