Blind Alleys loi 



because of its very uselessness, a source of danger to the or- 

 ganism. The assumption of special creation might imply 

 that this blind sac was put into man and into other animals 



Fig. 30. The Third Eyelid 



The little fold of tissue extending from the inner 

 corner of the eye has been called the " third eyelid," 

 although there is nothing about it to suggest an eye- 

 lid, in most of the familiar animals. In birds and in 

 certain reptiles and amphibians this membrane can be 

 drawn over the eye so as to cover it completely, but 

 among primates it is reduced to a negligible wrinkle 

 of skin, a, eye of ape; b, eye of owl; c, human eye; 

 c^ the semi-lunar fold, eye-ball removed. From 

 Gruenberg, Biology and Human Life, published by 

 Ginn & Company. 



simply as a means of misleading him in his efforts to under- 

 stand the world in which he lives. 



Another useless structure which finds its readiest ex- 

 planation in the supposition that there is a basic kinship 

 among various orders of life is the semi-lunar fold (Fig. 30) . 

 Here again the obvious " fitness " of the structure in some 

 groups of animals and its obvious superfluity in other 

 groups challenges us to explain its presence on some the- 



