Fig. 13-13. Primitive vertebrates, according to some zoologists, had fin folds much like amphioxus (upper 

 figure). Portions of these folds were retained in the pectoral and pelvic regions to become the paired 

 appendages; other parts became the other fins (middle and lower fish figures). Coming out on land 

 necessitated greater development of the appendages, as is seen among the amphibians, reptiles, and 

 mammals. At first they dragged their bodies over the ground, later the appendages supported the 

 body above the ground, which made it possible for the animal to move more swiftly. This reaches its 

 peak in such cursorial animals as the horse. 



