110 MORPHOLOGICAL DEVELOPMENT. 



typal vertebra ; and that the vertebrate animal is composed 

 of archetypal vertebrae arranged in a series, and sever- 

 ally modified to fit their positions these facts, I say, so far 

 from proving as much, suffice, when impartially considered, 

 to disprove it. No assigned nor any conceivable attribute of 

 the supposed archetypal vertebra is uniformly maintained. 

 The parts composing it are constant neither in their num- 

 ber, nor in their relative positions, nor in their modes of 

 ossification, nor in the separateness of their several individu- 

 alities when present There is no fixity of any one element, 

 or connexion, or mode of development, which justifies even a 

 suspicion that vertebrae are modelled after an ideal pattern. 

 To substantiate these assertions here would require too much 

 space, and an amount of technical detail wearisome to the 

 general reader. The warrant for them will be found in a 

 criticism on the osteological works of Prof. Owen, originally 

 published in the British and Foreign Medico- Chirurgical 

 Review for Oct. 1858. This criticism I add in the Appendix, 

 for the convenience of those who may wish to study the 

 question more fully. (See Appendix B.) 



Everything, then, goes to show that the segmeutal compo- 

 sition which characterises the apparatus of external relation 

 in most vertebrata, is not primordial or genetic, but function- 

 ally determined or adaptive. Our inference must be that the 

 vertebrate animal is an aggregate of the second order, in 

 which a relatively superficial segmentation has been pro- 

 duced by mechanical intercourse with the environment. "We 

 shall hereafter see that this conception leads us to a consist- 

 ent interpretation of the facts shows us why there has 

 arisen such unity in variety as exists in every vertebral 

 column, and why this unity in variety is displayed under 

 countless modifications in different skeletons. 



211. Glancing back at the facts brought together in 

 these two chapters, it seems probable that there has gone on 

 among animals a process parallel to that which we saw reason 



