VESTIGIAL ORGANS 261 



condition and can be of very little if any use to its possessor, 

 and there is reason to fear that in man himself the teeth are 

 disappearing a good deal more rapidly than we could wish, 

 though in this case the disappearance seems to be chiefly due 

 to disease. 



A better example of a vestigial structure in man is the 

 coccyx at the hinder end of the vertebral column (Fig. 95), which 

 j/represents the last remnant of the ancestral tail, and is occasion- 

 ally accompanied by vestiges of the muscles by which an ordinary 

 tail is moved. The hair on the chest, again, is a vestige of the 



V 



FIG. 116. Section through the vestigial Pineal Eye of the Frog (Tadpole) 



X 168. (From a photograph.) 

 epd., epidermis ; p.e., pineal eye; r.s., roof of skull ; v.n., vestigial stalk and nerve. 



hairy coat which once clothed the entire human body, as it still 

 does that of the apes. 



Examples of vestigial organs might be multiplied to an 

 indefinite extent, but enough has perhaps been said to show that 

 such structures are of very common occurrence and to indicate 

 their value as evidences of organic evolution. They occur, of 

 course, not only in the adult condition but also, as in the case of 

 the embryonic gill-slits of air-breathing vertebrates, at earlier 

 stages ofaeveTopment, but such cases will be more conveniently 

 dealt with in the next chapter. 



Closely akin to the occurrence of vestigial structures is another 

 phenomenon known as atavism or reversion, by which we mean 



