CHAP BER LY: 
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. 
