Mutation of Organisms 213 



from simpler orders of animals is the presence 

 of what are known as non-functional vestigial 

 structures, relics of past phases of existence 

 such, for instance, as the unused external 

 muscles of our ears and rudimentary third eye- 

 lids, the gill clefts of reptiles, birds, and 

 mammals, and the hind limbs in whales. The 

 study of these vestigial structures is of import- 

 ance in showing that ancestral features have a 

 great power of hereditary persistence. Our 

 firm conviction is, that the origin of variations 

 in the structure of organisms is, in every case, 

 due to the power which their protoplasmic basic 

 substance possesses of responding to the action 

 of the environment. Those organisms which 

 thus become most perfectly adapted to external 

 and internal stimuli, are those which survive in 

 the struggle for existence. 



