RUDIMENTARY ORGANS. 13 
but which 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 auricular 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 
