The body of man and the processes of life, in the earlier part of the century, were almost as unknown to most people as were the structure of the earth and the great processes of nature. What was known of human anatomy and physiology was contained in ponderous treatises, written in difficult and technical language suitable only for students of medicine and doctors. It was thought to be not only unnecessary but slightly coarse for those not in the profession to know anything of the viscera of digestion, circulation, and so forth. Huxley laid low this great superstition by his _Elementary Lessons in Physiology_, a little volume first published in 1866, which ran through many editions. In it he wrote primarily for teachers and learners in boys' and girls' schools, and selected from the great bulk of knowledge and opinion called human physiology only the important and well-established truths. So successful was he in his selection that, notwithstanding the immense increase in knowledge since he wrote, the book still remains an adequate and useful elementary treatise, and by this time must have given their main knowledge of the human body to hundreds and thousands of readers who otherwise would have remained ignorant. The books of which we have been writing were addressed to the general public, but, in addition, Huxley wrote several, of which three are specially important, for those students who devote themselves specially to anatomy. _The Crayfish_, his famous volume in the International Scientific Series, has been called by Professor Howes, the assistant and successor of Huxley at the Royal College of Science, "probably the best biological treatise ever written." Many naturalists have written elaborate monographs on single animals: Lyonet worked for years on the willow caterpillar, Strauss Durckheim devoted an even minuter attention to the common cockchafer, and the great Bojanus investigated almost every fibre in the structure of the tortoise. The volumes produced by these anatomists were valuable and memorable, and occupy an honoured place in the library of science, but Huxley's aim was wider and greater. He showed how careful study of one of the commonest and most insignificant of animals leads, step by step, from every-day knowledge to the widest generalisations and the most difficult problems of zoölogy. He made study of a single creature an introduction to a whole science, and taught students to regard any form of life not merely as a highly complicated and deeply interesting anatomical study, but as a creature that is only one out of an innumerable host of living things, every fibre in its body, every rhythm in its functions proclaiming the degree and nature of its relationship to other animals. R. Louis Stevenson, writing of his native town, tried to give "a vision of Edinburgh, not as you see her, in the midst of a little neighbourhood, but as a boss upon the round world, with all Europe and the deep sea for her surroundings. For