VERTEBRATE ANIMALS 231 



life, are connected, through amphibious beings that present almost 

 imperceptible gradations of development, 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 Vertebrata, the transition to the still more 

 intelligent and highly-endowed Mammals is effected with equal 

 facility, so that the zoologist finds, to his astonishment, that 

 throughout this division of animated nature, composed of crea- 

 tures widely differing among themselves in form and habits, a 

 series of beings, unbroken as regards their physical organization, 

 is distinctly traceable. 



The first grand character that distinguishes the Vertebrate 

 classes is the possession of an internal jointed skeleton, which is 

 endowed with vitality, nourished by blood-vessels, capable of 

 growth, and which undergoes a perpetual renovation by the re- 

 moval and replacement of the substances that enter into its 

 composition. 



In the lowest tribes of Vertebrata, the texture of the internal 

 framework of the body is permanently cartilaginous, and it con- 

 tinues through life in a flexible and consequently feeble condition ; 

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

 active and forcible movements, earthy particles are deposited in 

 the interstices of the cartilaginous substance, and, as these accu- 

 mulate, additional firmness is bestowed upon the skeleton, until it 

 becomes converted into perfect bone. 



The complete skeleton of a Vertebrate animal may be con- 

 sidered as being composed of several sets of bones, employed for 

 different purposes, consisting of a central portion, the basis and 

 support of the rest, and of various appendages derived from or 

 connected with the central part. The centre of the whole osseous 

 fabric is generally made up of a series of distinct pieces arranged 

 along the axis of the body; and this part of the skeleton is invari- 

 ably present ; but the superadded appendages, being employed in 

 different animals for very various and distinct purposes, present 

 the greatest diversity of form, and are many of them wanting in 

 any given genus; so that a really complete skeleton, that is, a 

 skeleton made of all the pieces which might normally enter into 

 its composition, does not exist, inasmuch as it is owing to the 

 deficiency of some portions, and the development of others in 

 particular races, that we must ascribe all the endless diversity of 



