EVOLUTION 921 



FURTHER ILLUSTRATION OF EVIDENCES OF 



EVOLUTION 



VESTIGIAL OR RUDIMENTARY STRUCTURES— In many 

 organisms, plants as well as animals, there are abundant illustra- 

 tions of more or less useless structures which are evidently the 

 survivals of better developed ones seen in ancestral types, or in 

 related species which are indicative of these. Darwin compared them 

 to the functionless or unsounded letters in words like leopard and 

 debt — words whose history would be more difficult to read if they 

 were spelt lepard and det. He compared them also to the functionless 

 buttons and buttonholes and many other structures in man's 

 clothing, whose historical significance is easily recognised. Others 

 have compared them to useless survivals, such as sinecure offices 

 in social organisations. As the word rudiment is also used to denote 

 the beginning of an organ (the primordium, or German Anlage), it 

 is clearer to keep the term vestigial, which suggests a trace or relic, 

 for more or less functionless and relatively minute structures which 

 are functional and well- developed in related types. It is desirable to 

 say "more or less" functionless; not only because small organs, like 

 the pituitary body, which were once regarded as useless, are now 

 known to be of great physiological importance, but also because 

 it is very difficult to prove that any living structure is quite 

 useless. 



Examples.— («) The third eyelid in man and monkeys is a 

 small tag of connective tissue in the inner upper corner of the eye. 

 It sometimes includes a minute piece of cartilage, the plica semi- 

 lunaris, and it is larger in some races than in others. It is a persistent 

 vestige of the third eyelid or nictitating membrane, which is well- 

 developed in most mammals (as also in birds and reptiles), — a 

 mobile structure that is used in cleaning the front of the eye. In 

 man and other Primates, this third eyelid is no longer needed for 

 this function, for the upper eyelid has become much more mobile 

 than in other mammals. In Cetaceans, again, the third eyelid is 

 absent, which may be correlated with the fact that the front of the 

 eye is continually washed with water. A well-developed third eyelid 

 may be seen in most common mammals, such as rabbits and dogs; 

 it may be readily observed in birds, flicking across the eyeball. 



[h] In the ear-trumpet or pinna of man there are vestigial muscles 

 — the relics of those that move the ear of many a mammal, such as 

 the horse. In most individuals these muscles are never activated; 

 yet it is not very unusual to find a man who, beginning young, is 

 able to move his ears. The dwindling is connected, directly or 

 indirectl}^ with the mobility of man's head, so that the location of 

 a sound is determined by a movement of the head as a whole. 



