An archetype endo-skeleton means that perfect model in which is arranged a succession of the vertebral segments, with their various processes, foramina, and appendages. Although, of course, no true typical vertebrate animal exists, such an image or model is not the less important; because it enables us to recognise, both in the skeletons of different animals as well as in the different segments of the same animal, those points or processes of bone which are strictly speaking homologous. The term "homologue" is thus defined by Professor Owen: — "The same organ in different animals, under every variety of form and function." But we use the term "analogue "to express a part or organ in one animal, which has the same functions as another part or organ in a different animal. The two terms have very different meanings, as the following illustrations will exemplify : The wings of a bird and the fore-limbs of the lion are strictly homologous parts, but their functions are not analogous. ,They are homologous, because composed of essentially similar structures, namely, humerus, radius and ulna, carpus, metacarpus, and phalanges, surrounded by muscles for flexion, extension, supination, and pronation, etc.; but they are not analogous, because the wings are employed in raising and sustaining the bird in the atmosphere, and the fore-limbs of the lion are constructed chiefly for seizing, holding, and tearing its prey. The shark supports itself in the rarer strata of the water by means of slow, yet powerful, strokes of its muscular tail; the eagle and the hawk do the same with their wings. These two parts, then, wings and tail, perform analogous functions, i. e. both are organs of propulsion; but they are not in themselves homologous, because the pectoral fin, and not the tail, is the repetition of the wing of the bird.
" Homologous parts are always indeed analogous parts in one sense, inasmuch as, being repetitions of the same parts of the body, they bear in that respect the same relation to different animals. But homologous parts may be, and often are, also analogous parts in a fuller sense, namely, as performing the same functions: thus, the fin or pectoral limb of the porpoise is homologous with that of a fish, inasmuch as it is composed of the same or answerable parts; and they are the analogues of each other, inasmuch as they have the same relation of subserviency to swimming. So, likewise, the pectoral fin of the flying-fish is analogous to the wing of the bird, but unlike the wing of the dragon (Draco volans), it is also homologous with it.*
" Relations of homology are of three kinds: the first is that above defined, viz. the correspondency of a part or organ, determined by its relative position and connexions, with a part or organ in a different animal; the determination of which homology indicates that such animals are constructed on a common type: when, for example, the correspondence of the basilar process of the human occipital bone with the distinct bone called ' basi-occipital,' in a fish or crocodile is shewn, the special homology of that process is determined.
* On the Homologies of the Vertebrate Skeleton, vol. i., p. 7.
" A higher relation of homology is that in which a part, or series of parts, stands to the fundamental or general type, and its enunciation involves and implies a knowledge of the type on which a natural group of animals, the vertebrate for example, is constructed. Thus, when the basilar process of the human occipital bone is determined to be the ' centrum' or ' body' of the last cranial vertebra, its general homology is enunciated.
"If it be admitted that the general type of the vertebrate endo-skeleton is rightly represented by the idea of a series of essentially similar segments succeeding each other longitudinally from one end of the body to the other, such segments being for the most part composed of pieces similar in number and arrangement, and though sometimes extremely modified for special functions, yet never so as wholly to mask their typical character; then any given part of one segment may be reproduced in the skeletons of different species, and this kind of repetition or representative relation in the segments of the same skeleton is called ' Serial Homology." As, however, the parts can be namesakes only in a general sense, as centrums, neurapophyses, ribs, etc.; and since they must be distinguished by different special names according to their particular modifications in the same skeleton, as e. g. mandible, coracoid, pubis, etc, such serially related or repeated parts are called homotypes."