THE LAWS OF ADAPTATION 



155 



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It must be the similarity of the internal physicochemical 

 energies of protoplasm, the similarity in the mechanics of 

 motion, of offense and defense, together with the constant simi- 

 larity of selection, which under- 

 lies the law of convergence or 

 parallelism in adaptation, name- 

 ly, the production of externally 

 similar forms in adaptation to 

 similar external natural forces, a 

 law which escaped the keen ob- 

 servation of Huxley^ in his re- 

 markable analysis of the modes 

 of vertebrate evolution pub- 

 lished in 1880. 



The whole process of motor 

 adaptation in the vertebrates, 

 whether among fishes, amphib- 

 ians, reptiles, birds, or mam- 

 mals, is the solution of a series 

 of mechanical problems, namely, 

 of adjustment to gravity, of 

 overcoming the resistance of 

 water or air in the develop- 

 ment of speed, of the evolution 

 of the limbs in creating levers, 

 fulcra (joints), and pulleys. 

 The fore and hind fins of fishes 

 and the fore and hind limbs of mammals evolve uniformly 

 where they are hemodynamic and divergently where they are 

 heterodynamic. This principle of homodynamy and hetero- 

 dynamy applies to the body as a whole and to every one of its 



"Huxley, T. H., 1880. 



Fig. 41. Convergent Adaptation of 

 Form in Three Wholly Unrelated 

 Marine Vertebrates. 



Analogous evolution of the swift-swim- 

 ming, fusiform body type (upper) in 

 the shark, a fish; (middle) in the 

 ichthyosaur, a reptile; and (lower) in 

 the dolphin, a mammal — three wholly 

 unrelated animals in which the in- 

 ternal skeletal structure is radically 

 different. After Osborn and Knight. 



