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 Saururæ, or reptile-like, birds, and contained a single fossil form, Archæopteryx, 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. Huxley's work upon birds, like his work in many other branches of anatomy, has been so overlaid by the investigations of subsequent zoölogists that it is easy to overlook its importance. His employment of the skeleton as the basis of classification was succeeded by the work of others who made a similar use of the muscular anatomy, of the intestinal canal, of the windpipe, of the tendons of the feet, and many other structures which display anatomical modifications in different birds. The modern student finds that all these new sets of facts are much greater in bulk than the work of Huxley, and it is easy for him to remain in ignorance that they were all suggested and inspired by the method which Huxley employed. He finds that further research has supplanted some of Huxley's conclusions, and it is easy for him to remain in ignorance that the conclusions themselves suggested the investigations which have modified them. Huxley's anatomical work was essentially living and stimulating, and too often it has become lost to sight simply because of the vast superstructures of new facts to which it gave rise. Closely associated with vertebrate anatomy is the subject of geographical distribution. In 1857 the study of this important department of zoölogy was placed on a scientific basis, practically for the first time, by a memoir on the geographical distribution of birds published in the _Journal_ of the Linnæan Society of London. It was known in a general way that different kinds of creatures were found in different parts of the world, but little attempt had been made to map out the world into regions characterised by their animal and vegetable inhabitants, as the political divisions of the world are characterised by their different governments and policies. Mr. Sclater, who two years later became secretary of the Zoölogical Society of London, in his memoir introduced the subject in the following words: