534 



a less yielding central axis. On the other hand, for tl/e central 

 axis to have become firmer while remaining continuous, would have 

 entailed a stiffness incompatible with the creature's movements. 

 Hence, increasing density of the central axis necessarily went hand 

 in hand with its segmentation: for strength, ossification was re- 

 quired ; for flexibility, division into parts. The production of ver- 

 tebra) resulting thus, there obviously would arise among them a 

 general likeness, due to the similarity in their mechanical conditions, 

 and more especially the muscular forces bearing on them. And then 

 observe, lastly, that where, as in the head, the terminal position and 

 the less space for development of muscles, entailed smaller lateral 

 bendings, the segmentation would naturally be less decided, less 

 regular, and would be lost as we approached the front of the 

 head. 



But, it may be replied, this hypothesis does not explain all the 

 facts. It does not tell us why a bone whose function in a given 

 animal requires it to be solid, is formed not of a single piece, but by 

 the coalescence of several pieces, which in other creatures are sepa- 

 rate ; it does not account for the frequent manifestations of unity of 

 plan in defiance of teleological requirements. This is quite true. 

 But it is not true, as Professor Owen argues respecting such cases, 

 that " if the principle of special adaptation fails to explain them, and 

 we reject the idea that these correspondences are manifestations of 

 some archetypal exemplar, on which it has pleased the Creator to 

 frame certain of his living creatures, there remains only the alterna- 

 tive that the organic atoms have concurred fortuitously to produce such 

 harmony." This is not the only alternative : there is another, which 

 Professor Owen has overlooked. It is a perfectly tenable supposi- 

 tion that all higher vertebrate forms have arisen by the superposing of 

 adaptations upon adaptations. Either of the two antagonist cosmo- 

 gonies consists with this supposition. If, on the one hand, we con- 

 ceive species to have resulted from acts of special creation ; then it 

 is quite a fair assumption that to produce a higher vertebrate animal, 

 the Creator did not begin afresh, but took a lower vertebrate animal, 

 and so far modified its pre-existing parts as to fit them for the new 

 requirements ; in which case the original structure would show itself 

 through the superposed modifications. If, on the other hand, 

 we 'conceive species to have resulted by gradual differentiations 

 under the influence of changed conditions; then, it would mani- 

 festly follow that the higher, heterogeneous forms, would bear 

 traces of the lower and more homogeneous forms from which they 

 were evolved. 



Thus, besides finding that the hypothesis of an " ideal typical 

 vertebra " is irreconcilable with the fac^s, we find that the facts are 

 interpretable without gratuitous assumptions. The average com- 

 munity of form which vertebras display, is explicable as resulting 



