EVOLUTION OF ANIMALS 



519 



ently either ancestral to those which possess the vestiges or are related 

 to such ancestors these parts and organs are fully developed. Among 

 the vestigial structures in man are the existence of supernumerary mam- 

 mary glands; the persistence of hair on the body; the presence in rare 

 cases of vestiges of a tail; the existence of a third eyelid, or nictitating 

 membrane; the presence of vestigial muscles, particularly in connection 

 with the ears; and the possession of a vermiform appendix. The last, in 



major muscle 



Colon 



^Auricularis 

 Superior muscle 



Ileum 



Auriculans 

 posterior 

 muscle 



Cartilage 



Antitragicus \ 1 

 muscle ^ 



A 



¥ 5 . . 



^ Auri culari s 



anterior muscle 



Helicis minor 

 muscle 



Tragi cos 

 muscle 



Caecuni 

 Appendix 



Pharyngeal 

 slits 



Ear- 



Fore limb 



Hind 

 limb 



Sacrum 



Sacra- ^ \/ Curvafores 

 coccygeal coccygis 



Tail ligaments d 'muscle 



Fig. 317. — Vestigial structures in man. A, the muscles of the ear, displayed by removal 

 of the superficial tissues. B, appendix, seen from behind. C, embryo, showing the tail. 

 £), abnormal persistence of tail muscles in adult, seen from behind. (Figs. B, C, and D 

 from Romanes, "Darwin and After Darwin," part I, by the courtesy of The Open Court 

 Publishing Company; Fig. A compiled from works on human anatomy.) 



many mammals, is a functional part of the intestine, adding to its capac- 

 ity; especially is this true of herbivorous forms. 



Many facts from comparative emhrijologij also support the idea of 

 evolution. Prominent among them are the examples of the biogenetic 

 law to which references have been made previously. The embryos of 

 different classes of vertebrates resemble each other very closely in early 

 stages, and differences begin to appear at points which may be assumed 

 to be where ancestral lines have diverged (Fig. 318). Among the most 

 striking of such differences are those which mark the separation of ter- 

 restrial from aquatic vertebrates. Other evidences are the stages in the 

 evolution of the circulatory system, including the changes in the branchial 



