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314 died in 1875 at the age of seventy-eight. The most important portions of Lyell's work were done after he had passed forty years; complete and sweeping revisions and enlargements of his earlier work were done late in life, and even down to within three days before his death, at the age of seventy-eight years, he finished a revision of his 'Principles of Geology,' a work which amazed and electrified scientists of all nations, and remains to-day the unchallenged great text-book in that field. Lyell's is the broadest and best-balanced mind which has dealt with deep-lying geological problems. In effect, he may be said to have created the science of geology. His work marked the second epoch in the thought of mankind, supplying the needed second link in the chain of evidence of planetary evolution. He applied in geology the principle of gradual development to the earth's crust, which Laplace and Kant had previously wrought in astronomy concerning sun systems and planets; which Darwin accomplished afterward in biology for living forms and organic life, and Spencer achieved for psychology in human consciousness and thought, and for sociology in human society and government. And, moreover, the fuller amplification of Lyell's work was made after he had passed sixty years.

With Lyell's work planetary evolution came to be a recognized and definite truth ; and then came Charles Darwin. Darwin was born in 1809, and lived until the age of seventy-three. His lifelong habits of thought, and his methods of research are too well known to be repeated, but it may be said that up to the age of forty-nine years he devoted himself almost wholly to accumulating stores of experience and observation, and to the planning of the great work which was to come afterward. 'The Origin of Species,' written at the age of fifty, sounded the farthest depth of biological knowledge and created such a whirlwind of controversy as no other book has done. His 'Descent of Man,' written at the age of sixty-two, was not less remarkable, and had an effect almost as widespread and profound. No man then living, either young or old, had the preparation, patience in the working out of details, breadth of mind, modesty or the honest simplicity of character, necessary to the carrying out of his tremendous task. Darwin may not have created the science of biology, but unmistakably he brought it out of a vague, confusing and conflicting state, reduced the mass of evidence and details to concrete form, and made it into an orderly and perfect system.

Herbert Spencer, the latest of this remarkable group of investigators, died in 1904, at the age of eighty-three. Spencer's mind did not begin its best functions until he was well on into the forties. He was storing up until then — his mind was incubating, as it were. At forty he had made merely a rough outline or program of his 'Synthetic Philosophy,' which massive work he was to carry out triumphantly in