614 THE WONDER OF LIFE 



coot (Perameles) which has five. Now a study of the 

 development of the teeth in shrews led Augusta Arnback- 

 Christie-Linde to the very interesting discovery, that 

 there are more than three incisor germs in both jaws of 

 Sorex araneus, and probably also in the upper jaw of 

 Neomys. These extra incisor germs in the Shrew are 

 apparently useless relics vestigial structures without 

 any function. They come and they go without attaining 

 full development. ' They are ', the discoverer says, 

 ' undoubtedly inherited from distant ancestors, which 

 consequently were to be found among polyprotodont (and 

 heterodont) mammals '. As regards the number of 

 incisor teeth it seems as if the Shrews bridged the gap to 

 which we alluded above. In any case, these extra incisor 

 germs seem to illustrate our present point of the long 

 lingering of structural relics which have outlived their use. 



In the inner upper corner of our eye there is a minute 

 half-moon-shaped fold, the plica semilunaris, a most 

 interesting item in the museum of relics which we carry 

 about with us in our body. For it corresponds to the 

 third eyelid (in whole or in part) which is well- developed 

 in most mammals and helps to clean the eye. It is vestigial 

 not only in Man, but in Monkeys and in Cetaceans. Its 

 practical absence in the Cetaceans is compensated for by 

 the continuous washing of the eye with water. In the 

 other cases the frequent movements of the upper eyelid 

 must make up for the vestigial state of the third eyelid. It 

 is a very old structure, a venerable relic, for it is the ' nicti- 

 tating membrane ' that is flicked across the eyes of Birds 

 and it is also represented in most Keptiles. 



It may be profitable to pursue the matter a little further. 

 The plica semilunaris sometimes includes in man a minute 



