88 EVOLUTION AND GENETICS 



true tails, but he records two cases in which they contained verte- 

 brae and were very probably vestigial remnants of the tails nor- 

 mally found in quadrupeds. 



The Third Eyelid. The eye is the seat of another vestigial 

 structure which normally persists. This is the thin pink fold at the 

 inner corner called the plica semilunaris, supposed to be a remnant 

 of the third eyelid of birds and amphibia. The full development 

 of the structure can easily be seen by watching an owl close its 

 eyes. As the upper and lower lids approach each other, the third 

 lid, a filmy, grayish membrane, moves across the eyeball behind 

 them. 



Vestigial Muscles. In a few parts of the human body muscles 

 persist which are usually useless and only rarely functional, among 

 them the muscles which move the ears and scalp. These are usu- 

 ally vestigial, but a few individuals retain the ability to contract 

 them at will. 



The Appendix. One of the most familiar vestiges is the vermi- 

 form appendix. Essentially the same in structure as the intestine, 

 it varies in different individuals from a tubular diverticulum to a 

 solid structure, and in length from three quarters of an inch to 

 nine inches. It is attached to the caecum, a blind end of the large 

 intestine which extends beyond the union of that tract and the 

 small intestine. Both caecum and vermiform process are found in 

 animals below man, and in some species, particularly herbivorous 

 animals, the caecum often attains relatively enormous dimensions 

 and is a correspondingly important organ. According to Wieder- 

 sheim it may be longer than the entire body, although in Carniv- 

 ora and several other orders it is reduced as greatly as in man. 



Wisdom Teeth. The third molars, or wisdom teeth, are like- 

 wise vestigial. Although the total number of thirty-two teeth is 

 less than the primitive number, we are losing four more, as is 

 apparent from the variable development of the wisdom teeth and 

 their usual ineffectiveness. 



The Significance of Vestigial Structures. In these and many 

 other structures a condition is visible in man which can be inter- 

 preted only as the persistence of parts for which he has lost or is 

 losing all need. Throughout the various organic systems of the 

 body more or less conspicuous examples of this type of resem- 

 blance may be found, and in all systems a very evident similarity 

 of minute structure, arrangement of tissue, and general plan of 



