CHAPTER V 



THE CONQUEST OF THE LAND 



THE Amphibia are the oldest and the lowest group of vertebrates 

 that are able to lead an active existence on land, and the characters 

 which distinguish them most definitely from the fishes are all 

 to be interpreted as adaptions to the new mode of life. One of 

 the most obvious external differences between the two groups 

 is in the structure of the extremities, the fish having fins, while 

 the amphibian has limbs constructed on the same general lines 

 as our own arms and legs. The fish's fin is to be regarded as an 

 extremity with a very great number of fingers or toes. It has 

 the function of a paddle, and is obviously useless whether for 

 supporting or propelling the body on land. The first obvious 

 necessity for a land existence is some mechanism by which the 

 limb can be alternately pushed forward and, being fixed to some 

 solid object, drawn upon, so as to pull the body after it. A 

 different arrangement of bones and muscles, so as to give a much 

 more complex lever system than that of a fin, and some kind 

 of clawing arrangement at the end, were thus necessary. The 

 similarity in the limbs of all the land vertebrates is very striking, 

 as is indicated by the comparison of human and a frog's limbs 

 on Fig. 84. In each case there is a single bone in the upper 

 arm or thigh, which is attached to a bony girdle in the trunk. 

 There are two elements in the forearm and in the lower leg re- 

 spectively, below which, in either case, is a group of small bones 

 constituting a complex joint at the wrist or ankle. Then follows 

 the set of five bones in the foot or hand, to each of which is 

 attached a jointed finger or toe. We have no reason to believe 

 that this particular arrangement, and the particular number of 



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