CHAPTER XI 



INTRODUCTION TO TETRAPODA : AMPHIBIA 



The two preceding chapters have dealt with vertebrates adapted to a 

 swimming existence. The remaining members of the phylum — grouped 

 together under the general name Tetrapoda — are on the other hand 

 adapted for progression upon a solid substratum. 



The primitive form of body of the tetrapod is probably represented 

 fairly closely by such a creature as a Newt (Fig. 177, A). The pectoral 

 limbs have become shifted back from the head region, the intervening 

 space forming a neek, and the hinder portion of the body extends back- 

 wards into a tail which remains of the protocercal type. The most 

 characteristic feature, however, of the Tetrapoda is that expressed in 

 their name, the limbs being in the form not of fins but of legs, terminating 

 in feet which are subdivided up into radiating toes or digits, normally 

 five in number. Nothing is definitely known as to the evolutionary 

 origin of this type of limb. The fashionable argument has been that 

 terrestrial vertebrates have evolved out of fish and that therefore legs 

 have evolved out of fins : and many attempts have been made to 

 show precisely how this has come about and how the various skeletal 

 elements in a leg are to be homologized with those in the paired fin 

 of a fish. Such attempts have not led to any general agreement and 

 in fact they appear to be based upon fallacious reasoning, the probability 

 being that existing terrestrial vertebrates have not evolved out of any 

 one .of the existing "fish" types, in which the paired limbs are 

 highly specialized for swimming. The probability would rather appear 

 to be that the typical fish and the typical tetrapods of to-day have 

 evolved along diverging lines of specialization from a common ancestral 

 type of primitive vertebrate in which the limbs were neither highly 

 specialized for swimming (fins) nor highly specialized for movement on a 

 solid substratum (legs). These unspecializcd limbs were in all probability 

 not far from the simple styliform type still seen to-day in the young 



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