Much of this the beginner might well postpone, his attention being solely drawn to the external characters; though of course those that are internal are by no means to be subsequently neglected. Indeed no satisfactory progress can be made in the serious study of Ornithology, or the Science of Birds, without a competent knowledge of their Anatomy and Development; while, though at present comparatively few fossil remains of Birds have been found, some of them are of the highest importance, and there is every probability of future discoveries throwing much light not only on the mutual relationships of Birds among themselves, but also on their connexion with the _Reptilia_. Birds are, in fact, only extremely modified Reptiles, the two Classes forming the _Sauropsida_ of Huxley, one of his three primary divisions of Vertebrata. {2}The aid of the Palaeontologist and Geologist must thus be called in to clear up many problems which present themselves to the Ornithologist who does not content himself with examining existing forms of life alone. _Archaeopteryx_ (p. 23) from the Jurassic System is the oldest Bird known, nor are any other pre-Tertiary forms recorded, save a small number from the rocks of the Cretaceous Epoch, the chief of which are the so-called _Odontornithes_, or toothed species of America (p. 45). The following paragraphs on the structure of Birds will help to explain the systematic account in the later chapters. FEATHERS.–Returning to the outward character denoted by the popular saying with which we began, the Feathers[2] constituting the plumage may not inconveniently be first considered. The general belief that they grow from almost every part of a Bird's body, as do hairs in most Mammals, is erroneous; for, almost without exception, they grow in certain definite tracts called _pterylae_, the intervening spaces, whether they be wholly bare or covered with down, being termed _apteria_. The arrangement of these patches is at times of considerable assistance in determining a Bird's affinities; and the subject may be studied in Nitzsch's _Pterylographie_[3] or in a shorter form in Dr. Gadow's article "Pterylosis" in Professor Newton's _Dictionary of Birds_.