CLASSIFICATION AND DISTRIBUTION 



365 



animals which appear quite unlike are really built upon the same 

 plan of structure, differing only in the manner that the plan is 

 carried out. For example, the frog possesses a spinal column 

 made of vertebrae, and two pairs of legs attached to the body by 

 girdles, each containing a certain number of bones. The rabbit 

 (Fig. 145) has a skeleton based upon the same type. It also 

 possesses a spinal column made of vertebra, with two pairs of 

 appendages attached by girdles to the axis of the body; and each 

 appendage is made up of bones which can be compared, bone 

 by bone, with those in the appendages of the frog. If Figure 145 

 is compared with Figure 88, this similarity can be seen and fol- 

 lowed out in very close detail, nearly all of the bones of the frog 

 being represented in the skeleton of the rabbit. This similarity 

 is found in spite of the fact that the two animals are so unlike 

 in general appearance and in habits. One lives in the water and 

 uses its legs for swimming and hopping; the other lives on the 



land, using its legs for 



support. But although 



., ,.. 



built for different pur- 

 poses, the skeleton of 

 these two animals is 

 evidently based upon 

 the same plan. Figure 

 146 shows another ex- 

 ample of similarity in 

 structure representing 

 the hand of man, C, 

 and the corresponding 

 fore foot of four other 

 animals. Although the 

 hand of man is used for a totally different purpose from that 

 of the fore legs of the horse, the ox, or any of the other animals 

 represented, it is evident that they are built upon the same plan 

 of structure. In each there are a radius and ulna, and a series of 

 wrist and finger bones. There are differences, it is true: while 



The skeleton of the hand of man, C; and the fore feet 

 of a horse, A; a rhinoceros, B; a pig, D; and an ox,E. 

 r, radius ; u, ulna. The other bones are not named but 

 may be easily compared. 



