186 THE GENESIS OF SPECIES. [CHAP. 



Nevertheless, in tliose species which, taken together, con- 

 stitute a series of more and more distinctly segmented 

 forms, the segmentation gradually increases all around the 

 central part of the spinal column. 



Mr. Spencer 13 thinks it probable that the sturgeon has 

 retained the notochordal (that is, the primitive, unsegment- 

 ed) structure because it is sluggish. But Dr. Gunther in- 

 forms me that the sluggishness of the common tope ( G-aleus 

 vulgaris) is much like that of the sturgeon, and yet the 

 bodies of its vertebrae are distinct and well ossified. More- 

 over, the great salamander of Japan is much more inert and 

 sluggish than either, and yet it has a well-developed, bony 

 spine. 



I can learn nothing of the habits of the sharks Hexan- 

 chus, Heptanchus, and Echinorhinus, but Muller describes 

 them as possessing a persistent chorda dorsalis). 1 * It may 

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

 among the very swiftest and most active of fishes. 



In the bony pike (lepidosteus), 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 argument by the non-segmen- 

 tation of the anterior end of the skeletal axis, i. e., by the 

 non-segmentation of the skull. But in fact the skull is seg- 

 mented, and, according to the quasi-vertebral theory of the 

 skull put forward by Prof. Huxley," is probably formed of 

 a number of coalesced segments, of some of which the tra- 

 beculae cranii and the mandibular and hyoidean arches are 

 indications. What is, perhaps, most remarkable, however, 



13 p r i nc ipi es o f Biology," vol. ii., p. 203. 



14 Quoted by H. Stannius in his " Handbuch der Anatomie der Wir- 

 belthiere," Zweite Auflage, Erstes Buch, 7, p. 17. 



15 In his last Hunterian Course of Lectures, 1869. 



