RUDIMENTARY ORGANS. 13 



but wliicli are always perfectly inactive. Our long-eared 

 ancestors of the tertiary period — apes, semi-apes, and 

 pouched animals, like most other mammals, moved their 

 large ear-flaps freely and actively; their muscles were much 

 more strongly developed and of great importance. In a 

 similar way, many varieties of dogs and rabbits, under the 

 influence of civilized life, have left off " pricking up " their 

 ears, and thereby have acquired imperfect amricular muscles 

 and loose-hanging ears, although their wild ancestors moved 

 their stiff" ears in many ways. 



Man has also these rudimentary organs on other parts of 

 his body ; they are of no importance to life, and never per- 

 form any function. One of the most remarkable, although 

 the smallest organ of this kind, is the little crescent-like fold, 

 the so-called "plica semilunaris," which we have in the 

 inner corner of the eye, near the root of the nose. This in- 

 significant fold of skin, which is quite useless to our eye, 

 is the imperfect remnant of a third inner eyelid which, 

 besides the upper and under eyelid, is highly developed in 

 other mammals, and in birds and reptiles. Even our very 

 remote ancestors of the Silurian period, the Primitive Fishes, 

 seem to have possessed this third eyelid, the so-called nicti- 

 tating membrane. For many of their nearest kin, who still 

 exist in our day but little changed in form, viz. many 

 sharks, possess a very strong nictitating membrane, which 

 they can draw right across the whole eyeball, from the inner 

 corner of the eye. 



Eyes which do not see form the most striking example of 

 rudimentary organs. These are found in very many animals, 

 which live in the dark, as in caves or underground. Their 

 eyes often exist in a well-developed condition, but they are 



