After working up the geological results of the long voyage in the _Beagle_, he set himself with great determination to more purely zoological details. While visiting the coast of Chili, he had found a curious new cirripede, to understand the structure of which he had to examine and dissect many of the common forms. The memoir, which was originally designed to describe only his new type, gradually expanded into an elaborate monograph on the Cirrepedes (barnacles) as a Whole group. For eight years he continued this self-imposed task, getting at last so weary of it as to feel at times as if the labour had been in some sense wasted which he had spent over it, and this suspicion seems to have remained with him in maturer years. But when at last the two bulky volumes, of more than one thousand pages of text, with forty detailed plates, made their appearance, they were hailed as an admirable contribution to the knowledge of a comparatively little known department of the animal kingdom. In the interests of science, perhaps, their chief value is to be recognised not so much in their own high merit as in the practical training which their preparation gave the author in anatomical detail and classification. He spoke of it himself afterwards as a valuable discipline, and Professor Huxley truly affirms that the influence of this discipline was visible in everything which he afterwards wrote. It was after Darwin had got rid of his herculean labours over the 'Cirripede book' that he began to settle down seriously to the great work of his life--the investigation of the origin of the species of plants and animals. One of the three volumes of the Biography is entirely devoted to tracing the growth of his views on this subject, and the preparation and reception of the great work on the _Origin of Species_. In no part of his task has the editor shown greater tact and skill than in this. From the earliest jottings, which show that the idea had taken hold of Darwin's mind, we are led onwards through successive journals, letters, and published works, marking as we go how steadily the idea was pursued, and how it shaped itself more and more definitely in his mind. It is impossible to condense this story within the limits of a Review article, and the condensation, even if possible, would spoil the story, which must be left as told in the author's own words. Briefly, it may be stated here that he seems to have been first led to ponder over the question of the transmutation of species by facts that had come under his notice during the South American part of the voyage in the _Beagle_--such as the discovery of the fossil remains of huge animals akin to, but yet very distinct from, the living armadillos of the same regions; the manner in which closely allied animals were found to replace one another, as he followed them over the continent; and the remarkable character of the flora and fauna of the Galapagos archipelago. 'It was evident,' he says, 'that