1386 THE STORY OF THE UNIVERSE 



admits of its assuming every degree and variety of 

 curvature. 



The mode in which the boa exerts a powerful pres- 

 sure on the bodies of the animals it has seized, and 

 which it has encircled within its folds, required the 

 ribs to be movable laterally, as well as backward, in 

 order to elude the force thus exerted. The broad 

 convex surfaces on which they play give them, in 

 this respect, an advantage which the ordinary mode 

 of articulation would not have afforded. The spinous 

 processes in this tribe of serpents are short and widely 

 separated, so as to allow of flexion in every direction. 

 In the rattlesnake, on the other hand, their length 

 and oblique position are such as to limit the upward 

 bending of the spinal column, although, in other re- 

 spects, its motion is not restricted. The vertebrae at 

 the end of the tail are furnished with broad trans- 

 verse processes for the attachment of the first joints of 

 the rattle. 



But of whatever variety of flexions we may sup- 

 pose the lengthened body of a serpent to be capable, 

 it will, at first view, be difficult to conceive how these 

 simple actions can be rendered subservient to the 

 purposes of progression on land : and yet experience 

 teaches us that few animals advance with more celer- 

 ity on the surface of the ground, or dart upon their 

 prey with greater promptitude and precision. They 

 raise themselves without difficulty to the tops of the 

 highest trees, and escape to their hiding-places with 

 a quickness which eludes observation and baffles the 

 efforts of their pursuers. 



The solution of this enigma is to be sought for 



