EVOLUTION-PAST AND PRESENT 



639 



•frontalis 



Quriculoris 



obicularis oculi 

 nasalis 



zyqomaticus 



piatysma 



Fig. 25-14. Vestigial structures such as ear muscles in man (auricularis) are similar to those in the monkey. In the 

 latter they have some function in moving the ears v/hich perhaps aids in directing the sound into the ear; in man 

 they are vestigial and serve no purpose, yet they persist. 



muscles in both animals are similar, most 

 of them performing the same function in 

 both. They vary only in size due to use, 

 which is reflected in the activity of the 

 animal. The cat walks on all fours and 

 therefore uses its front appendages pri- 

 marily for locomotion, whereas man puts his 

 homologous appendages to quite a different 

 use. Facial muscles of both man and mon- 

 key are likewise similar, although we have 

 more of them and consequently can register 

 more emotions in our faces (Fig. 25-14). 

 The digital tips of a bird, a man, and a 

 horse, referred to earlier (Fig. 14-2), can 

 be compared in the same way. Other ex- 

 amples were mentioned earlier in the dis- 

 cussion of the origin of scales, teeth, and 

 feathers ( see p. 369 ) . All of these compar- 

 able structures and many more support the 

 thesis of common origin, with modifications 

 in various species in response to function. 

 Common origin implies evolution. 



From vestigial structures 



Many animals possess structiu-es which 

 are vestiges of some past functional organ. 

 The appendix in man is such a structure. It 



apparently performed some function when 

 man's diet was not quite what it is today. 

 Perhaps at one time he was more of an 

 herbivore. In any case, it is no longer 

 needed, as is attested by continued health 

 after its removal. Indeed, failure to remove 

 an infected appendix may mean death in 

 man. It seems strange that this potential 

 danger has not been removed by selection! 

 Another human vestigial structure is the 

 ear muscle. Man possesses a complete set 

 of muscles, similar to those in other pri- 

 mates, for moving the ears in all directions 

 (Fig. 25-14). However, it is only with ex- 

 treme difficulty that he can move his audi- 

 tory appendages and even then tlie move- 

 ment is inefi^ective in improving his hearing. 

 Man also possesses a rudimentary tail to- 

 gether with a complete set of muscles for 

 wagging it in all directions. These are even 

 more useless than the ear muscles yet this 

 structure has persisted for thousands of 

 generations. Since there is wide variation in 

 the number of segments in the tail ( coccyx ) 

 one might believe that it is disappearing, 

 and this is probably true. A similar situation 

 exists with the last set of molars (wisdom 



