We have seen that some of the most important of the contributions made by Huxley to zoölogical knowledge were in the field of the lower animals, especially of those marine forms for the study of which he had so great opportunities on the _Rattlesnake_. A great bulk of his zoölogical work, however, related to the group of back-boned animals. These, by their natural affinities and anatomical structure, are more closely related to man, and, as Huxley began his scientific work as a medical student, the groundwork of all his knowledge was study of the anatomy and physiology of man. Moreover, throughout the greater part of his working life, he had more to do with the extinct forms of life. The vertebrate animals, from the great facility for preservation which their hard skeleton presents, as well as from the extremely important anatomical characters of the skeleton, bulk more largely in the study of palæontology than does any other group. In each of the great groups of vertebrate animals, in fishes, amphibia, reptiles, birds, and mammals, Huxley did important work. Much of this is embodied in his treatise on _Vertebrate Anatomy_, but to some particular parts of it special attention may now be directed, as much because these serve as excellent examples of his method of work as because of their intrinsic importance. The skull is the most striking feature in the skeleton of vertebrate animals, and to the theory and structure of the vertebrate skull Huxley paid special attention, and his views and summary of the views of others form the basis of our modern knowledge. This work was put before the public in the course of a series of lectures on Comparative Anatomy given in 1863, while Huxley was Hunterian Professor at the Royal College of Surgeons, and the beginnings of it were contained in a Croonian lecture to the Royal Society in 1858.