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 zooelogists.  Before Huxley, it had been attempted by a number of distinguished zooelogists; 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 zooelogist 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.  The immediate practical results were equally important.  He was able to shew that among the innumerable known forms there were three grades of structure.  The lowest had already been recognised and named by Haeckel; it consisted of the Saururae, or reptile-like, birds, and contained a single fossil form, Archaeopteryx, distinguished from all living birds by the presence of a hand-like wing in which the metacarpal bones were well developed and freely movable, and by the possession of a long lizard-like tail actually exceeding in length the remainder of the spinal column.  The next group of Ratites, although it contained only the Ostrich, Rhea, Emu, Cassowary, and Apteryx, he shewed to be equivalent in anatomical coherence to the third great group of Carinates, which includes the vast majority of living birds.  In his arrangement of the latter group, he laid most stress on the characters of the bony structures which form the palate, and by this simple means was able to lay down clearly at least the main lines of a natural classification of the group.