54 The Life Story of the Fish 



be attached to the principal element of the skeleton, the 

 backbone. As we have seen, the pectoral fins did come to be 

 fastened to the skull, but that was perhaps as much due to 

 the requirements of the gill apparatus, of which their sup- 

 porting bones formed the rear end, as to anything else. The 

 ventral fins in all the primitive fishes, including those still 

 alive today like the tarpon and the salmon family, had no 

 connection with the backbone. They were attached to pieces 

 of cartilage or bone, and these pieces reposed without sup- 

 port in the flesh of the body. 



When the fish came up out of the water and turned itself 

 into a land animal, the paired fins, which had been of no 

 great importance to it, turned into limbs, and became ex- 

 tremely important. Some method had to be found by which 

 they could be made to support the body, and to do this it 

 was obviously necessary to attach them to the main element 

 of the skeleton. At the front end this was accomplished very 

 cleverly. In quadrupeds the front end, with its weighty, 

 overhanging head and its ribs for the protection of the lungs 

 and heart, is the heavier and needs the most support. Using 

 the old bones which had braced the pectoral fins, a kind of 

 cradle was devised, itself resting on good solid bearings on 

 the front legs, from which the heavy weight was slung by 

 means of more or less elastic connections. The shoulder-blade 

 and its dependent muscles and ligaments are the principal 

 elements in this arrangement, and the whole thing is flexible, 

 so that the shocks received on the front feet when running 

 or jumping do not jar the whole body. 



The rear end was a tougher problem. Here there was 

 no existing connection, and in the very earliest land animal, 

 an amphibian known only through fossils, the fish system 

 still prevailed, and the hind limbs were not attached to the 

 backbone at all. It soon became obvious that that was not 

 going to be satisfactory, and a new system was designed. A 



