CHAPTER IV. 



VESTIGIAL STRUCTURES (continued}. 



THUS far a few clear examples of vestiges of organs and 

 parts have been briefly considered ; it may, perhaps, be 

 desirable to trace some such part from as near its 

 beginning as possible, through its various modifications 

 and complete disappearance as a part normal to a given 

 animal. It is also well known that many parts are 

 present in the embryo which are not represented in the 

 mature animal. Of this we had many instances in the 

 preceding chapter, but such parts are often persistent 

 as abnormalities in the adult ; they are then described 

 as being reversionary, or atavistic. As this is a subject 

 of great interest to us, the matter to be considered now 

 will serve as an excellent introduction to the chapter on 

 atavism. The next few pages will be devoted to the 

 description of cervical auricles in man and animals ; the 

 study is of interest in many ways, and especially from 

 the circumstance that it clearly shows that disuse is, in 

 itself, not able to bring about the complete disappear- 

 ance of parts. It will also serve to illustrate the em- 

 bryological rule that when parts are precociously 

 developed in the embryo, but feebly developed in the 

 adult, it indicates that they were of high importance in 

 the ancestors of those particular animals. 



