THE ANIMAL ANCESTRY OF MAN 67 



combined certain characters of the crossopt, or lobe- 

 finned fishes, with others of the dipnoan group, while avoiding 

 the pecuhar specializations of either. When these adven- 

 turous pioneers first pushed their way up on to the dry 

 land they were still using the old wriggling movements of 

 the body invented by the very earhest chordates. In some of 

 those forms in which the body was very long and the paired 

 paddles were relatively small, the wriggHng movements 

 greatly predominated and in several lines the incipient 

 paired hmbs became reduced and degenerate, thus giving 

 rise to snake-hke or eel-hke amphibians. In the Hues that 

 were more nearly related to our own ancestry, on the con- 

 trary, the fore and hind paddles, corresponding respectively 

 to our arms and legs, became larger and stronger, the internal 

 bony rods of the extremities, due to the new stresses of 

 terrestrial life, became shifted and modified into the highly 

 characteristic five-rayed hands and feet which were safe- 

 guarded by all the later stages in the hne of ascent to man. 

 Indeed man owes to these earliest amphibians the entire 

 ground-plan of his anatomy, including the skeletal and 

 muscular parts of his locomotor machinery. Beneath the 

 successive modifications acquired in adaptation to later 

 special life habits, man shares this tetrapodal ground-plan 

 with tens of thousands of other species of land-Hving verte- 

 brates of the great classes Amphibia, Reptiles, Birds and 

 Mammals, which are collectively bracketed as the super- 

 class Tetrapoda, or four-limbed animals. 



Let us consider a little more in detail the mechanism of the 

 tetrapod locomotor machiner}^ especially in so far as it has 

 served as 'a starting-point for that of man. Even in the 

 stage of the air-breathing fishes the simple arrangement of 

 zigzag muscle segments which had sufficed to produce the 

 wriggling movements of earlier forms had become com- 

 plicated, first by the outgrowths of humps of the body-wall 

 surmounted by folds of skin to serve as keels and rudders, 

 and secondly by the extension of buds from the zigzag 

 muscle plates into the bases of these primitive fins, enabling 

 the fish to warp them and finally to move them independently 

 of the general body movements. By the time of the lobe- 

 finned fishes the fore and hind pair of paddles had already 



