310 VERTEBRATA. 



tensive series, although, of course, modified in ac' 

 cordance with the endless diversity of circumstances 

 under which particular races are destined to exist. 



The immeasurable realms of the ocean, the rivers, 

 the 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 capacity 

 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 con- 

 duct us at last to Man himself. 



Fishes, restricted by their mode of respiration to an 

 aquatic life, are connected, through amphibious beings 

 that present almost imperceptible gradatioris of de- 

 velopment, with terrestrial and air-breathing Reptiles ; 

 these, progressively endowed with greater perfection 

 of structure and increased powers, slowly conduct us 

 to the active and hot-blooded Birds, fitted by their 

 strength and by the vigour of their movements to an 

 aerial existence. From the feathered tribes of ver- 

 tebrata, the transition to the still more intelligent 

 and highly-endowed Mammalia is effected with equal 

 facility, so that the zoologist finds, to his astonish- 

 ment, that, throughout this division of animated 

 nature, composed of creatures widely differing among 

 themselves in form and habits, a series of beings un- 

 broken as regards the physical organization, is dis- 

 tinctly traceable. 



The first grand character that distinguishes the ver- 

 tebrate classes, is the possession of an internal jointed 

 skeleton, which is endowed with vitality, nourished 

 by blood-vessels, capable of growth, and which under- 

 goes a perpetual renovation by the removal and 

 replacement of the substances that enter into its com- 

 position. 



In the lowest tribes of vertebrata, the texture of 

 the internal framework of the body is permanently 

 cartilaginous, and it continues through life in a flexi- 



