588 



:lifferences in the conrlitions of the respective vertebras nocessitate 

 dilTerences in their structures. 



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

 of the phenomena, is to recognize this general law m its couverso 

 apphcation. If vertebrge are unlil^e in proportion to the unlikeness 

 of tlieir circumstances, then, by imphcation, they will be like in pro- 

 portion 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 then* mechani- 

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

 know that throughout the Vertebrata generally, and in each vertebrate 

 animal, the vertebrae, amid all their varying cu-cumstances, 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 

 vertebrae, 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, mstead of comparing the vertebral columns of chffereut 

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

 recognize the same law. In the segments of an mdividual 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 struc- 

 ture 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 sul)stance. 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 Amphinxus, 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. 

 Ui'eater muscular endowments presupposed a firmer internal fulcrum 



