32 WHAT EVOLUTION IS 



heart of the fish. These embryonic 

 organs in man never serve for breath- 

 ing as the corresponding parts do in 

 fishes, but in gross structure the 

 human gill arches recall in a most 

 striking way the gill system of fishes. 

 As the development of the human 

 embryo proceeds, the gill clefts are 

 obliterated, excepting the first one 

 v^hich is retained in forming the aper- 

 ture of the external ear. 



Thus the frog and man and in fact 

 all the higher vertebrates show in a 

 temporary way gill clefts and gill 

 arches, both of which are the perma- 

 nent possessions of the fishes. 



That higher animals should, in the 

 course of their individual develop- 

 ment, exhibit temporarily features 

 that are permanent in lower animals, 

 seems to be a rule of organic growth. 

 It certainly is abundantly exempli- 

 fied in many forms. Thus in all 



