1390 THE STORY OF THE UNIVERSE 



points of support, are made successively to approach 

 and to separate from each other, the body being pro- 

 pelled by bringing it from a curved to a straight 

 line. 



There is yet a third kind of motion, which serpents 

 occasionally resort to, when springing upon their 

 prey, or when desirous of making a sudden escape 

 from danger. They coil themselves into a spiral, by 

 contracting all the muscles on one side of the body, 

 and then, suddenly throwing into violent action all 

 the muscles on the opposite side, the whole body is 

 propelled, as if by the release and unwinding of a 

 powerful spring, with an impulse which raises it to 

 some height from the ground, and projects it to a 

 considerable distance. 



Thus these animals, to which nature has denied all 

 external members, are yet capable, by the substitu- 

 tion of a different kind of mechanism, still con- 

 structed from the elements belonging to the primi- 

 tive type of vertebrated animals, of silently gliding 

 along the surface of the earth, of creeping up trees, 

 of striding rapidly across the plain, and of executing 

 leaps with a vigor and agility which astonish the be- 

 holder, and which, in ages of ignorance and super- 

 stition, were easily ascribed to supernatural agency. 



The conformation of those parts of the frame 

 which are subservient to progressive motion becomes 

 more perfect in the class of Saurian reptiles, which 

 includes all the lizard tribes. Several links of con- 

 nection with the preceding class may still be noticed, 

 marking the progress of development, as we follow 

 the ascending series of animals. Rudiments of the 



