186 THE GENESIS OF SPECIES. [Chap. 



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

 stitute a series of more and more distinctly segmented 

 forms, the segmentation gradually increases all arou?id the 

 central part of the spinal column. 



Mr. Spencer '* thinks it probable that the sturgeon has 

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

 ed) structure because it is sluggish. JJut Dr. Gllnther in- 

 forms me that the sluggishness of the connnon tope ( Galeics 

 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 Ilexan- 

 ohus, lleptanchus^ and Echinorhlnus^ but Midler describes 

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

 be tiiey 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. Wliat is, perhaps, most remarkable, however, 



13 ''Principles of Biology," vol. ii., p. 203. 



'^ Quoted by H. Stannius in his " Ilandbuch der Anatomic der Wir- 

 belthiere," Zweite Auflagc, Erstes Buch, § 7, p. 17. 

 '^ In his last Iluuterian Course of Lectures, 18G9, 



