Many reasons make it difficult for us to realise, now, the singular novelty and importance of Huxley's memoir on the Medusæ. The first is a reason which often prevents great discoveries in almost every subject from receiving in after years their due respect. The years that have passed since 1850 have seen not only the most amazing progress in our knowledge of comparative anatomy, but almost a revolution in the methods of studying it. Huxley's work has been incorporated in the very body of science. A large number of later investigators have advanced upon the lines he laid down; and just as the superstructures of a great building conceal the foundations, so later anatomical work, although it has only amplified and extended Huxley's discoveries, has made them seem less striking to the modern reader. The present writer, for instance, learned all that he knows of anatomy in the last ten years, and until he turned to it for the purpose of this volume he had never referred to Huxley's original paper. When he did so, he found from beginning to end nothing that was new to him, nothing that was strange: all the ideas in the memoir had passed into the currency of knowledge and he had been taught them as fundamental facts. It was only when he turned to the text-books of anatomy and natural history current in Huxley's time that he was able to realise how the conclusions of the young ship-surgeon struck the Fellows and President of the Royal Society as luminous and revolutionary ideas. In the first half of the century, a conception of the animal kingdom prevailed which was entirely different from our modern ideas. We know now that all animals are bound together by the bond of a common descent, and we seek in anatomy a clue to the degrees of relationship existing among the different animals we know. We regard the animal kingdom as a thicket of branches all springing from a common root. Some of these spring straight up from the common root unconnected with their fellows. Others branch repeatedly, and all the branches of the same stem have features in common. What we see in the living world is only the surface of the thicket, the tops of the twigs; and it is by examination of the structure of this surface that we reconstruct in imagination the whole system of branches, and know that certain twigs, from their likeness, meet each other a little way down; that others are connected only very deep down, and that others, again, spring free almost from the beginning. The fossils of beds of rock of different geological ages give us incomplete views of the surface of the thicket of life, as it was in earlier times. These views we have of the past aspects of the animal kingdom are always much more incomplete than our knowledge of the existing aspect; partly because many animals, from the softness of their bodies, have left either no fossil remains at all, or only very imperfect casts of the external surfaces of their bodies; and partly because the turning of any animal