During the voyage he had written long letters to his friends descriptive of what he had seen and done. He likewise forwarded considerable collections of specimens gathered by him at various places. His scientific activity was therefore well known at home to his acquaintances, and even to a wider circle, for some of his letters to Henslow were privately printed and circulated among the members of the Cambridge Philosophical Society. It would have been difficult for any even of his most intimate friends to offer a plausible conjecture as to the line of inquiry in natural science that he would ultimately select as the one along which he more particularly desired to advance. An onlooker might have naturally believed that the ardent young observer would choose geology, and end by becoming one of the foremost leaders in that department of science. In his _Journal of Researches_, and in the letters from the _Beagle_ now published, it is remarkable how much he shows the fascination that geology exercised upon him. He had thoroughly thrown off the incubus of Wernerianism. From Lyell's book and Sedgwick's personal influence he had discovered how absorbingly interesting is the history of the earth. Writing to his friend, W. D. Fox, from Lima, in the summer of 1835, he expresses his pleasure in hearing that his correspondent had some intention of studying geology; which, he says, offers 'so much larger a field of thought than the other branches of natural history'; and, moreover, 'is a capital science to begin, as it requires nothing but a little reading, thinking and hammering.' While the whole of his _Journal_ shows on every page how keen were his powers of observation, and how constantly he was on the watch for new facts in many fields of natural knowledge, it is to the geological problems that he returns most frequently and fully. And never before in the history of science had these problems been attacked by an actual observer over so vast a space of the earth's surface, with more acuteness and patience, or discussed with such breadth of view. There is something almost ludicrous in the contrast between his method of treatment of volcanic phenomena and that of his professor at Edinburgh only six short years before. But though geological questions, being the most obvious and approachable, took up so large a share of his time and attention, he was already pondering on some of the great biological mysteries the unveiling of which in later years was to be his main occupation, and to form the basis on which his renown as an investigator was chiefly to rest.