A DEFINITION OF EVOLUTION 



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Figure 16. Analogical Resemblance Between the Fohe Fin of a Shark (A) and 

 THE Fore Flipper of a Seal (B). Though siiperficiallv similar, tlie skeletons are very 

 different. 



resemblance, for each lias the form of a short, broad paddle. But this 

 adaptive form is achieved upon an entirely different basis in the two cases 

 (Figure f6). For the skeleton of the fin has a verv simple structure, 

 though perhaps similar to that which has given rise to the limbs of higher 

 vertebrates. But the skeleton of the flipper of any aquatic mammal is, for 

 all its adaptation to a mode of life unusual among the mammals, still \er\ 

 recognizably that of a mammal. The parts are strongl)' foreshortened, but 

 the typical bones are easily recognized, and thev still bear the same spatial 

 relationships to one another as in more typical mammals. Thus even ex- 

 treme adaptive changes have not becMi able to destroy the evidence of 

 true homologies. 



Again, the vertebrate eve is one of the most complex and (efficient \isual 

 organs in the animal kiniidom. Minor variations in the various vertebrate' 

 classes involve changes like those of the limb bones of the seal, that is, 

 identical parts differ in proportions and ma\ I unction somewhat differ- 

 ently. Thus the muscles of accommodation function in mammals by vary- 

 ing the tension upon (he suspcMisorv ligamcMit of ihe lens, and thus c-ontrol- 

 ling its curvature. In birds, more rapid accommodation is achieved by the 

 same muscles pulling the lens closer to the retina or by relaxation permit- 

 ting it to move farther away, in all \ crtebrates, the eye is homologous, 

 constructed of identical materials which are used in similar ways. The 

 most nearly sinnlar eye outside of the C^hordata is that of the Cephalopoda 



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