I was one day talking with Professor Owen in the Hunterian Museum, when a gentleman approached with a request to be informed respecting the nature of a curious fossil, which had been dug up by one of his workmen. As he drew the fossil from a small bag, and was about to hand it for examination, Owen quietly remarked:—“That is the third molar of the under-jaw of an extinct species of rhinoceros.” The astonishment of the gentleman at this precise and confident description of the fossil, before even it had quitted his hands, was doubtless very great. I know that mine was; until the reflection occurred that if some one, little acquainted with editions, had drawn a volume from his pocket, declaring he had found it in an old chest, any bibliophile would have been able to say at a glance: “That is an Elzevir;” or, “That is one of the Tauchnitz classics, stereotyped at Leipzig.” Owen is as familiar with the aspect of the teeth of animals, living and extinct, as a student is with the aspect of editions. Yet before that knowledge could have been acquired, before he could say thus confidently that the tooth belonged to an extinct species of rhinoceros, the united labours of thousands of diligent inquirers must have been directed to the classification of animals. How could he know that the rhinoceros was of that particular species rather than another? and what is meant by species? To trace the history of this confidence would be to tell the long story of zoological investigation: a story too long for narration here, though we may pause awhile to consider its difficulties.