xi THE ELEMENTS OF STRUCTURE 213 



When old-fashioned structures are turned to new uses, 

 as in the case already mentioned of the first gill-cleft 

 becoming the Eustachian tube, the term vestigial should 

 not be used, unless it be carefully qualified. Vestigial 

 structures in the strict sense are structures which linger 

 on in dwindled expression for ages after they have ceased 

 to be of use. They have often been compared to the 

 vestigial structures in clothes, buttons that have no 

 corresponding buttonholes, and holes that have no corre- 

 sponding buttons. 



Some of the Cetaceans show deeply buried remnants of 

 a hip-girdle and even of the upper part of the leg,- 

 remnants that are in certain cases very variable, as 

 vestigial organs are apt to be. In the spiracle of the 

 skate (see fig. 86) there is a minute vestige of a lost gill ; 

 the spiracle is in no sense a vestige, but the comb-like 

 ridge is. Of vestigial structures in man much has been 

 said, and Prof. Wiedersheim has brought the facts 

 together in his striking book, The Structure of Man, an 

 Index to his Past History. He shows that we are anti- 

 quarians even if we are unaware of it, and walking 

 museums of relics. In the median corner of the eye lies 

 the vestigial third eyelid, larger in some races than in 

 others, and sometimes with a minute supporting cartilage. 

 Now. this third eyelid or nictitating membrane is present 

 and well-developed in most mammals, not to speak of 

 birds and reptiles ; it is a useful structure which cleans 

 the front of the eye-ball when it is flicked across. It is 

 absent in Cetaceans, w r here the eye is continually washed ; 

 it is vestigial in apes and in man, where the mobility of 

 the upper eyelid has rendered the retention of the extra 

 structure unnecessary. Similarly, the muscle which 

 moves the trumpet or pinna of the ear in many mammals, 

 such as dog and donkey, is vestigial in man, though some 

 have it larger than others and can even call it into 

 activity by wasting sufficient attention on the senseless 

 effort. 



11. Adaptation in Organs. One of the outstanding 

 impressions that we get from the study of structure is 

 that of adaptiveness. The structure is always so admir- 



