VERTEBRATA. 477 



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 occu- 

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

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

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

 amphibious beings, that present almost imperceptible gradations of 

 developement, with terrestrial and air-breathing REPTILES : these, 

 progressively 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 MAM- 

 MALIA is effected with equal facility ; 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. 



(518.) 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, nou- 

 rished by blood-vessels and supplied with nerves, capable of 

 growth, and undergoing a perpetual renovation by the removal 

 and replacement of the substances that enter into its compo- 

 sition. 



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 in the interstices of the carti- 

 laginous substance, and, in proportion as these accumulate, addi- 

 tional firmness is bestowed upon the skeleton, until it assumes, 

 at length, hardness and solidity proportioned to the quantity of 

 the contained earthy matter, and becomes converted into perfect 

 bone. 



(519.) Phenomena precisely similar are observable in tracing the 

 formation and developement of the osseous system, even in those 



