There were various considerations which led me to regard the “petrified nail” in this case as one of the most interesting fossils I had ever seen; and, before quitting Orkney, to pursue my explorations farther to the south, I brought two intelligent geologists of the district,[5] to mark its place and character, that they might be able to point it out to geological visitors in the future, or, if they preferred removing it to their town museum, to indicate to them the stratum in which it had lain. It showed me, among other things, how unsafe it is for the geologist to base positive conclusions on merely negative data. Founding on the fact that, of many hundred ichthyolites of the Lower Old Red Sandstone which I had disinterred and examined, all were of comparatively small size, while in the Upper Old Red many of the ichthyolites are of great mass and bulk, I had inferred that vertebrate life had been restricted to minuter forms at the commencement than at the close of the system. It had begun, I had ventured to state in the earlier editions of a little work on the “Old Red Sandstone,” with an age of dwarfs, and had ended with an age of giants. And now, here, at the very base of the system, unaccompanied by aught to establish the contemporary existence of its dwarfs,—which appear, however, in an overlying bed about a hundred feet higher up,—was there unequivocal proof of the existence of one of the most colossal of its giants. But not unfrequently, in the geologic field, has the practice of basing positive conclusions on merely negative grounds led to a misreading of the record. From evidence of a kind exactly similar to that on which I had built, it was inferred, some two or three years ago, that there had lived no reptiles during the period of the Coal Measures, and no fish in the times of the Lower Silurian System.