3 o8 AMPHIBIA xn. 7- 



would be used as a lever, giving greater effect to the wave-like motions 

 by which the creature 'swam on land'. The muscles of the limb, con- 

 tracting in a serial manner, would tend to move it backwards and 

 forwards relative to the body, thus assisting in locomotion. At first 

 the limb perhaps carried only little weight, but as tetrapod evolution 

 proceeded the limbs became elongated and turned under the body, 

 raising it off the ground. To work effectively in this way the limbs 

 came to be held bent down at elbow and knee (Fig. 181) and a firm 

 application to the ground was produced by bending outwards at 

 wrist and ankle. Finally the limbs were brought in to the side of the 



Fig. 182. Suggested protetrapod stage, between crossopterygian and labyrinthodont. 

 (From Oregon - and Raven.) 



body by rotation, such that the elbow pointed backward and the knee 

 forward. 



These are the changes that must have occurred at some time to 

 produce the full tetrapod condition, but we cannot follow exactly the 

 order in which they took place. Their effect is to convert a paddle- 

 like fin, whose main movements were up and down, and were used 

 for stabilization in the horizontal swimming plane, into an elongated 

 jointed strut, on which the animal can balance, and which can be 

 moved as a lever to produce locomotion. 



The limbs and girdles and their muscles show a remarkable con- 

 stancy of pattern throughout the tetrapods. The muscles of the fins of 

 fishes are concerned mainly with lowering and raising (Fig. 192), and 

 they run from a girdle in the body wall to the basal radials in the fin, 

 and between the radials. After the animals came on land the muscles 

 served not only to raise and lower the limbs but also to draw them 

 forwards and backwards; indeed, many fishes already make such 

 movements, including Protopterns. The muscles therefore become 

 arranged around the shoulder and hip joints into groups serving as 

 adjustable braces, by which the body is balanced on its legs and by 

 whose contraction the latter are moved. Those muscles that draw 

 the limb towards and away from the mid-ventral line can be called 



