ON THE VERTEBRATE SKELETON. 339 



would have been more proper to have signified such serial homology by giving 

 the general term applicable to such parts, as abstract vertebral elements. 



The fact is, however, that the mastoid (s) is the parapophysis of its verte- 

 bra, whilst the ilium is a portion of the pleurapophysis of its vertebra ; and 

 the mastoid is serially homologous with the transverse process (parapophysis) 

 of a sacral vertebra (fig. 27, p), not with the pleurapophysis or ' ilium ' ; it 

 is not, therefore, a repetition of the ilium in the skull. The true expression 

 of the ideas which suggested the terms ' ilium of the head,' ' scapula of the 

 head,' &c, will be found in the true enunciation of the serial homologies of 

 the vertebrate skeleton. 



It finally remains for inquiry, admitting the explanation of the endoskeletal 

 archetype given in this Report to be the true one, whether such is the 

 ultimate attainable generalization, or whether we may not also gain an in- 

 sight into the nature of the force by which all the modifications of the 

 vertebrate skeleton, even those subservient to the majesty of man himself, 

 are still subordinated to a common type. 



We perceive in the fact of the endoskeleton consisting of a succession 

 of segments similarly composed, — in the very power, in short, of enunciating 

 special, general and serial homologies, — an illustration of thatlaw of vegetative 

 or irrelative repetition which is so much more conspicuously manifested by 

 the segments of the exoskeleton of the invertebrata, as, for example, in the 

 rings of the centipede and worm, and in the more multiplied parts of the 

 skeletons of the echinoderms. 



The repetition of similar segments in a vertebral column, and of similar 

 elements in a vertebral segment, is analogous to the repetition of similar cry- 

 stals as the result of polarizing force in the growth of an inorganic body. 



Not only does the principle of vegetative repetition prevail more and more 

 as we descend in the scale of animal life, but the forms of the repeated parts 

 of the skeleton approach more and more to geometrical figures; as we see, 

 for example, in the external skeletons of the echini and star-fishes : nay, the 

 calcifying salt actually assumes in such low-organized skeletons the very 

 crystalline figures which characterize it when deposited, and subject to the 

 general polarizing force, out of the organized body. Here, therefore, we 

 have direct proof of the concurrence of such general and all-pervading polar- 

 izing force with the adaptive or special organizing force in the development 

 of an animal body. 



The. marvellous phaenomena of this development have, hitherto, been ex- 

 plained by two hypotheses or forms of expression, as the result, viz. of ' vital 

 properties' either peculiar to living matter or common to all, but latent in 

 dead, matter ; or, as due to the operation of one or more ' vital principles,' 

 vital forces, dynamies or faculties, answering to the iceai of Plato, deemed 

 by that philosopher to be superadded to matter and mind, and which he de- 

 fined as a sort of models, or moulds in which matter is cast, and which 

 regularly produce the same number and diversity of species*. 



Now besides the Ilea, organizing principle, vital property, or force, which 

 produces the diversity of form belonging to living bodies of the same materials, 

 which diversity cannot be explained by any known properties of matter, there 

 appears also to be in counter-operation during the building up of such bodies 

 the polarizing force pervading all space, and to the operation of which force, 

 or mode of force, the similarity of forms, the repetition of parts, the signs 

 of the unity of organization may be mainly ascribed. 



The platonic Ilea or specific organizing principle or force would seem to 

 * Sec Barclay, Life and Organization, 8vo, 1822. 



