observations. At this moment came the third of the great turning points in his life, which as a mysteriously disguised blessing was brought about through ill health. In London he was entering official duties and public scientific service which would undoubtedly have increased and interfered more and more seriously with his work. We can only count it as one of the most fortunate circumstances in the history of science that Darwin at the age of thirty-three was forced to leave London and to move to Down. Here for forty years he never knew for one day the health of an ordinary man; his life was one long struggle against the strain of sickness. But unrealized by him there was the compensation of a mind undisturbed by the constant interruption of outside affairs, such interruption as killed Huxley and is killing so many fine and ambitious men to-day. When I saw Huxley and Darwin side by side in 1879, the one only fifty-four, the other seventy, the younger man looked by far the more careworn of the two. Huxley, the strong man, broke down mentally at fifty-six; Darwin, the invalid, was vigorous mentally at seventy-two.
Darwin's writings fall into three grand series. In the nine years after he returned from the voyage, or between his twenty-seventh and thirty-sixth years, Darwin wrote the first series, including his pre-evolutionary geological and zoological works, his "Coral Reefs" (1842), his "Zoology and Geology of the Voyage of the Beagle" (1844-1846), his "Journal of Researches," the popular narrative of his voyage (1845). Darwin's ill health thereafter shut him off from geology, although his last volume, "The Earthworm," was in a sense geological.
It is characteristic of the life of every great man that his genius and his own self-analysis instinctively guide him to discover his mental needs.
Until the age of forty-five Darwin in his own opinion had not completed his education, in the sense that education is a broad and exact training. He now proceeded to fill the one gap in his training by devoting the eight years of his life, between thirty-seven and forty-five, to a most laborious research upon the barnacles, or Cirripedia. This gave him the key to the principles of the natural or adaptively branching and divergent arrangement of animals through the laws of descent as set forth in the "Origin," which he certainly could not have secured in any other way. The value he placed on his work on the barnacles is of especial import to-day when systematic work is so lightly esteemed by many biologists, young and old. Darwin subsequently, in the words of Hooker, "recognized three stages in his career as a biologist, the mere collector at Cambridge, the collector and observer on the Beagle, and for some years afterwards, and the trained naturalist after, and only after, the Cirripede work."