1396 THE STORY OF THE UNIVERSE 



grees of relative development, and converting them, 

 by these transformations, into a mechanism of a very 

 different kind, and subservient to other objects than 

 those to which they are usually applied. It is 

 scarcely possible to have stronger proofs, if such 

 were wanting, of the unity of plan which has regu- 

 lated the formation of all animal structures than 

 those afforded by the skeleton of the tortoise. 



The first step taken to secure the relative immo- 

 bility of the trunk is to unite in one rigid, bony 

 column all its vertebrae, and to allow of motion only 

 in those of the neck and of the tail. The former, 

 accordingly, are all anchylosed together, leaving, in- 

 deed, traces of their original forms as separate verte- 

 brae, but exhibiting no sutures at the place of junc- 

 tion. The canal for the spinal marrow is preserved, 

 as usual, above the bodies of these coalesced verte- 

 brae, and is formed by their united leaves; the arches 

 being completed by the spinous processes. But these 

 processes do not terminate in a crest as usual; they 

 are further expanded in a lateral direction, forming 

 flat pieces along the back, which are united to one 

 another by sutures, and which are also joined to the 

 expanded ribs, so as to form the continuous plane 

 surface of the carapace. The transverse processes of 

 the vertebrae are well marked, but, though firmly 

 united to the ribs, do not give rise to them; for the 

 ribs, which are flattened and expanded, so as to touch 

 one another along their whole length, are inserted 

 below, between the bodies of every two adjoining 

 vertebrae ; while above they are united by suture with 

 the plates of the spinous processes. This change in 



