limits of the ossification of the primitive cartilages differ in extent, but occur in homologous situations. This primitive skull is roofed over by a series of membrane bones which have no connection in origin with the other portions of the skull, and which have no representative in the vertebral column, but which are the direct descendants of the bony scales clothing the external skin in cartilaginous fishes. In one respect only was Huxley erroneous. Partly by inadvertence, and partly because the minute details of vertebrate embryology became really familiar to zoölogists only after the elaborate work of Balfour of Cambridge, Huxley, in his account of the formation of the first beginnings of the skeleton in the embryo, made confusion between the walls of the primitive groove, which, in reality, give rise to the nervous structures, and those embryonic tissues which form the skeletal system. The next great piece of work which we may take as typical of Huxley's contributions to vertebrate anatomy, is his classical study on the classification of birds. The great group of birds contains a larger number of species than is known in any other group of vertebrates, and, in this vast assemblage of forms there is strikingly little anatomical difference. The ostrich and the humming-bird might perhaps be taken as types of the extremest differences to be found, and yet, although these differ in size, plumage, adaptations, habits, mode of life, and almost everything that can separate living things, the two conform so closely to the common type of bird structure that knowledge of the anatomy of one would be a sufficient guide, down to minute details, for dissection of the other. None the less, there are hundreds of thousands of species of birds between these two types. It is not surprising that to reduce this vast assemblage of similar creatures to an ordered system of classification has proved one of the most difficult tasks attempted by zoölogists. Before Huxley, it had been attempted by a number of distinguished zoölogists; but, for the most part, these had relied too much on merely external characters and on superficial modifications in obvious relation to habits. When Huxley, in the course of a set of lectures on Comparative Anatomy, was about to approach the subject of birds he was asked by a zoölogist how he proposed to treat them. "I intend," he replied, "to treat them as extinct animals." By that he meant that it was his purpose to make a prolonged study of their skeletal structures the basis of his grouping, following the lines which Cuvier, Owen, and he himself had pursued so successfully in the case of the fossil remains of vertebrates. The result was that this first systematic study of even one set of the anatomical characters of the group completely reformed the method by which all subsequent workers have tried to grapple with the problem; ornithology was raised from a process akin to stamp-collecting to a reasoned scientific study.