22 WHAT EVOLUTION IS 



is abundantly illustrated by many 

 sets of organs. For instance, the 

 human arm is composed of parts that 

 recur in the corresponding organs in 

 other animals. The arm of man, as 

 shown on page 23, contains four sets 

 of bones : the single bone of the upper 

 arm, the pair of bones in the forearm, 

 the group of small wrist bones, and 

 the series of elongated bones in the 

 five digits. All these groups of bones 

 recur with great regularity in the 

 foreleg of the cat, of the turtle, and 

 of even so lowly organized an animal 

 as the frog. The wing of a bat, when 

 it is examined, is found not to be con- 

 structed upon a plan peculiar to itself, 

 but to be a modification of the type of 

 structure already described for man, 

 in that the single bone of the upper 

 arm is present, as are the pair of 

 forearm bones, the wrist bones, and, 

 enormously elongated to carry the 



