ON VARIATION. 73 



already noted. In the deserts of Asia, South Africa, 

 and North America some of the Hzards exhibit a great 

 elongation of the lateral scales of the digits on one or 

 both extremities. These become fringes of spines, 

 freely articulated at their bases with the mtegument. 

 By penetrating the sand, they increase the hold on its 

 yielding surface, and greatly improve the speed of 

 their movements. The genera in which this structure 

 is conspicuous in the three localities in question, be- 

 long to as many distinct families. Thus in Asia it is 

 the genus Phrynocephalus of the family Agamidae ; in 

 South Africa it is Ptenopus of the Gecconidae ; while 

 in North America it is Uma of the Iguanidae. I give 

 figures of the feet of Ptenopus and Uma for comparison. 

 (Fig. 23.) Phrynocephalus is more like Uma than is 

 Ptenopus. 



In the succeeding chapters of this book many illus- 

 trations of the serial relation of characters will be 

 given, so that it is not necessary to occupy more space 

 with the subject here. 



