606 VERTEBKATA. 



CHAPTER XXV. 



VERTEBRATA. 



(1614.) THE fifth division of the animal kingdom is composed of four 

 great classes of animals, closely allied to each other in the grand fea- 

 tures of their organization, and possessing in common a general type of 

 structure clearly recognizable in every member of the extensive series, 

 although, of course, modified in accordance with the endless diversity of 

 circumstances under which particular races are destined to exist. The 

 immeasurable realms of the ocean, the rivers, lakes, and streams, the 

 fens and marshy places of the earth, the frozen precincts of the poles, 

 and the torrid regions of the equator, have all appropriate occupants, 

 more favoured as regards their capacities for enjoyment, and more largely 

 endowed with strength and intelligence, than any which have hitherto 

 occupied our attention, and gradually rising higher and higher in their 

 attributes, until they conduct us at last to Man himself. PISHES, 

 restricted by their organization to an aquatic life, are connected by 

 amphibious beings, that present almost imperceptible gradations of 

 development, with terrestrial and air-breathing REPTILES : these, pro- 

 gressively attaining greater perfection of structure and increased powers, 

 slowly conduct us to the active, hot-blooded BIRDS, fitted by their 

 strength, and by the vigour of their movements, to an aerial existence. 

 From the feathered tribes of Vertebrata, the transition to the still more 

 intelligent and highly-endowed MAMMALIA is effected with equal faci- 

 lity ; so that the anatomist finds, to his astonishment, that throughout 

 this division of animated nature, composed of creatures widely differing 

 among themselves in form and habits, an unbroken series of beings is 

 distinctly traceable. 



(1615.) The first grand character that distinguishes the vertebrate 

 classes is the possession of an internal jointed skeleton, which is not, 

 as in the preceding classes, extravascular and incapable of increase 

 except by the successive deposition of calcareous laminae applied to its 

 external surface, but endowed with vitality, nourished by blood-vessels 

 and supplied with nerves, capable of growth, and undergoing a per- 

 petual renovation by the removal and replacement of the substances 

 that enter into its composition. 



(1616.) In the lowest tribes of aquatic Vertebrata the texture of the 

 internal framework of the body is permanently cartilaginous, and it 

 continues through life in a flexible and consequently feeble condition ; 

 but as greater strength becomes needful, in order to sustain more active 

 and forcible movements, calcareous particles are found to be deposited 



