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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 phenomena 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 idéac 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 i¢éa, 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 i¢éa or specific organizing principle or force would seem to 
: * See Barclay, Life and Organization, 8vo, 1822. © 
* 
