THE LIMBS OF LIZARDS 139 



specialised, by means of natural selection acting upon 

 ever-occurring morphological variations. 



It naturally follows that locomotion in lizards will 

 be of different kinds, according to the state of develop- 

 ment or absence of limbs. The two extremes are, of 

 course, the quick, running movements of the lizards 

 with perfect fore and hind limbs, and the quiet, 

 gliding motion of the entirely limbless forms, like the 

 slow-worm. In intermediate forms the method of 

 progression is likewise modified, sometimes exhibiting 

 bipedal locomotion, the hind-Hmbs only being used. 

 If the four limbs are very small in proportion to 

 the body, the locomotion is correspondingly sluggish. 

 Along with the variations in the limbs themselves 

 there have occurred alterations in the muscular 

 structures to enable the reptile to adopt the new 

 mode of locomotion, and other organs have become 

 similarly modified for their particular functions in the 

 changed conditions of life. 



It is only necessary to study the respective move- 

 ments of different kinds of lizards to appreciate what 

 a wonderful amount of specialisation has taken place 

 in connection with this function alone, and the reader 

 who would pursue this interesting topic is recom- 

 mended to turn his attention to the geckos, skinks, 

 and the slow-worm, to observe the immense variety 

 of movements which have resulted from structural 

 modifications. He will then naturally turn his atten- 

 tion to the question of the relationship of the lizards 



