174 



EVOLUTION AND ANIMAL LIFE 



homology, and that it is the representation of the current of 

 heredity. The resemblances arise from common origin, the 



FIG. 106. Ears of various anthropoid apes and of man, showing human vestigial 

 characters: 1, hairy human ear; 2, Barbary ape; 3, chimpanzee; 4 and 5, human 

 ears; 6, ear of human fretus; 7, orang-outang. 



variations from the demand of differing external conditions. It 

 may be said that the inside of an animal tells what it is, the out- 

 side where it has been. In the internal structure, ancestral 

 traits are perpetuated with little change 

 through geologic ages. The external 

 characters affected by every feature of 

 the surroundings may be rapidly altered 

 through response to demands of environ- 

 ment and through the destruction of in- 

 dividuals whose life fails of adjustment. 



It is in the persistence of heredity 

 that we find the explanation of vestigial 

 organs. An organ well developed in one 

 group of animals or plants may in some 

 other be reduced to an imperfect organ 

 or rudiment so incomplete as to serve 

 no purpose whatever. Such rudimentary 

 or functionless structures may be found 

 in the body of any of the higher animals and in most or all 

 of the higher plants. As a rule such structures are more fully 





FIG. 107. Head of a five- 

 months human embryo 

 showing embryonic hair- 

 covering. (After Ecker.) 



