THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 123 



nor in the separateness of their several individualities when 

 present. There is no fixity of any one element, or con- 

 nexion, 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 Re- 

 view for Oct. 1858. This criticism I add in the Appendices, 

 for the convenience of those who may wish to study the 

 question more fully. ( See Appendix B. ) 



Everything, then, goes to show that the segmental 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 con- 

 sistent 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. On glancing back at the facts brought together in 

 these two chapters, we see it to be probable that there has gone 

 on among animals a process like that which we saw reason 

 to think has gone on among plants. Minute aggregates of 

 those physiological units which compose living protoplasm, 



* A qualifying fact should be named. When the production of vertebral 

 segments has become constitutionally established, so that there is an innate 

 tendency to form them, there arises a liability to form supernumerary ones ; 

 and this, from time to time recurring, may lengthen the series, as in the 

 body of a snake or the neck of a swan. This qualification, however, affects 

 equally the hypothesis of an ideal type and the hypothesis of mechanical 

 genesis. 



