564 APPENDIX B. 



differences in the conditions of the respective vertebrae necessitate 

 differences in their structures. 



Now, it seems to us that the first step towards a right concep- 

 tion of the phenomena, is to recognize this general law in its converse 

 application. If vertebras are unlike in proportion to the unlike- 

 ness of their circumstances, then, by implication, they will be like in 

 proportion to the likeness of their circumstances. While successive 

 segments of the same skeleton, and of different skeletons, are all in 

 some respects more or less differently acted on by incident forces, 

 and are therefore required to be more or less different ; they are 

 all, in other respects, similarly acted on by incident forces, and are 

 therefore required to be more or less similar. It is impossible to 

 deny that if differences in the. mechanical functions of the vertebrae 

 involve differences in their forms ; then, community in their mechani- 

 cal functions, must involve community in their forms. And as we 

 know that throughout the Vertebrata generally, and in each vertebrate 

 animal, the vertebras, amid all their varying circumstances, have a 

 certain community of function, it follows necessarily that they will 

 have a certain general resemblance — there will recur that average 

 shape which has suggested the notion of a pattern vertebra. 



A glance at the facts at once shows their harmony with this 

 conclusion. In an eel or a snake, where the bodily actions are such 

 as to involve great homogeneity in the mechanical conditions of the 

 vertebras, the series of them is comparatively homogeneous. On the 

 contrary, in a mammal or a bird, where there is considerable hetero- 

 geneity in their circumstances, their similarity is no longer so great. 

 And if, instead of comparing the vertebral columns of different 

 animals, we compare the successive vertebrae of any one animal, we 

 recognize the same law. In the segments of an individual spine, 

 where is there the greatest divergence from the common mechanical 

 conditions ? and where may we therefore expect to find the widest 

 departure from the average form ? Obviously at the two extremities. 

 And accordingly it is at the two extremities that the ordinary 

 structure is lost. 



Still clearer becomes the truth of this view, when we consider the 

 genesis of the vertebral column as displayed throughout the ascend- 

 ing grades of the Vertebrata. In its first embryonic stage, the spine 

 is an undivided column of flexible substance. In the early fishes, 

 while some of the peripheral elements of the vertebrae were marked 

 out, the central axis was still a continuous unossified cord. And 

 thus we have good reason for thinking that in the primitive verte- 

 brate animal, as in the existing Amphioxus, the notochord was per- 

 sistent. The production of a higher, more powerful, more active 

 creature of the same type, by whatever method it is conceived to 

 have taken place, involved a change in the notochordal structure. 

 Greater muscular endowments presupposed a firmer internal fulcrum 



