ON THE VERTEBRATE SKELETON. 1 75 



in the ' Philosophic Anatomique ' (8vo, 1818, 4ieme memoire, p. 205), that 

 the branchial arches of fishes are the modified tracheal rings of the air- 

 breathing vertebrates: we perceive at once that he is enunciating a relation 

 of homology. 



I have elsewhere* discussed the relations, both homological and analogical, 

 of the respiratory organs of the air-breathing and water-breathing vertebrate 

 animals, and have here adverted to them merely to illustrate the essential 

 distinction of those relations. In the ' Glossary ' appended to the first volume 

 of my ' Hunterian Lectures,' the terms in question are denned as follows : — 



" Analogue." — A part or organ in one animal which has the same func- 

 tion as another part or organ in a different animal. 



" rloMOLOGUE." — The same organ in different animals under every variety 

 of form and functionf." 



The little ' Draco volans ' offers a good illustration of both relations. Its 

 fore-limbs being composed of essentially the same parts as the wings of a bird 

 are homologous with them; but the parachute being composed of different 

 parts, yet performing the same function as the wings of a bird, is analogous 

 to them. 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, viz. as perform- 

 ing the same functions : thus the fin or pectoral limb of a Porpoise is homo- 

 logous 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, 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 connections, 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 shown, the special homology of that process is determined. 



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 func- 

 tions, yet never so as to wholly mask their typical character, — then any 

 given part of one segment may be repeated in the rest of the series, just as 

 one bone may be reproduced in the skeletons of different species, and this 

 kind of repetition or representative relation in the segments of the same 

 skeleton I call ' serial homology.' As, however, the parts can be namesakes 

 only in a general sense, as centrums, neurapophyses, ribs, &c ; and since 



* Lectures on Vertebrata, 1846, p. 279. 



f Lectures on Invertebrate Animals, 8vo, 1843. Glossary, pp. 374, 379. My ingenious 

 and learned friend Mr. Hugh Strickland has made a strong and able appeal to the good 

 sense of comparative anatomists in favour of the restriction of these terms to the senses in 

 which they are here defined.— Phil. Mag. 1846, pp. 358, 362. 



