194 THE GENESIS OF SPECIES. [CHAP. 



may be they have the habits of the tope, but other sharks 

 are amongst the very swiftest and most active of fishes. 



In the bony pike (hpidosteus), the rigidity of the bony 

 scales by which it is completely enclosed must prevent 

 any excessive flexion of the body, and yet its vertebral 

 column presents a degree of ossification and vertebral 

 completeness greater than that found in any other fish 

 whatever. 



Mr. Spencer supports his mechanical hypothesis by the 

 non-segmentation of the anterior end of the skeletal axis, 

 i.e. by the non- segmentation of the skull. But in fact the 

 skull is segmented, and, according to the quasi-vertebral 

 theory of the skull put forward by Professor Huxley, 1 is 

 probably formed of a number of coalesced segments, of 

 some of which the trabecula? cranii and the mandibular 

 and hyoidean arches are indications. What is perhaps 

 most remarkable, however, is that the segmentation of the 

 skull its separation into the three occipital, parietal, and 

 frontal elements is most complete and distinct in the 

 highest class, and this can have nothing, however remotely, 

 to do with the cause suggested by Mr. Spencer. 



Thus, then, there is something to be said in opposition 

 to both the aggregational and the mechanical explanations 

 of serial homology. The explanations suggested are very 

 ingenious, yet repose upon a very small basis of fact. 

 Not but that the process of vertebral segmentation may 

 have been sometimes assisted by the mechanical action 

 suggested. 



It remains now to consider what are the evidences in 

 support of the existence of an internal power, by the action 

 of which these homological manifestations are evolved. It 



1 In his last Hunterian Course of Lectures, 1869. 



