To appreciate this minuteness and thoroughness, it is necessary to examine the less known and less popular books, such as the ‘Cross and Self Fertilization’ and the ‘Different Forms of Flowers.’ In these one is overwhelmed with Darwin’s persistence in examining and noting trivial details. And perhaps most impressive of all is to turn over the pages of the two immensely solid volumes on Cirripedes. For many years Darwin devoted himself to the study of these unexciting barnacles, sometimes wearying, sometimes rebelling, but always keeping at his task until he had completed it. He himself sometimes wondered whether such prolonged toil at mere description was wholly worth while; but Huxley believed that the mental discipline was of the greatest possible profit to Darwin’s later work. In any case the exhaustive thoroughness of it is indeed exemplary. How far this goes may be suggested by one quotation out of many: ‘I cannot too strongly impress on any one intending to study this class, not to trust to external characters; he must separate and clean and carefully examine the internal structure and form of the compartments and more especially of the opercular valves.’[37] And the examination, in Darwin’s case, applied to hundreds of specimens of minute barnacles gathered and sent to him from all parts of the world. It is hardly necessary to emphasize the enormous amount of labor implied and involved in all these self-imposed tasks of Darwin. Although circumstances compelled him to give a large part of his life to repose, he was by nature a worker. It is true that he sometimes speaks jokingly of his idleness: ‘I have been of late shamefully idle, _i.e._, observing instead of writing, and how much better fun observing is than writing.’[38] But as Huxley well points out, Darwin generally means by idleness ‘working hard at something he likes when he ought to be occupied with a less attractive subject.’[39] And Darwin’s own more serious comment is, ‘I am a pretty man to preach, for I cannot be idle, much as I wish it, and am never comfortable except when at work.’[40] He complains of fatigue, he forces himself to seek recreation, relaxation; but even when his body is at rest, his mind tends to work, refuses to stop working, finds its only real relief in change of occupation and thought.