CREATION BY EVOLUTION
If we examine the angle of a cat's eye corresponding to the nasal angle in man we shall see there not a small fold of membrane but a veritable third eyelid. This additional lid may be made to pass under the two outer lids and over the whole exposed surface of the eyeball in a way to sweep this surface completely. It is provided with muscles by which this movement is quickly and easily made. This nictitating membrane, as it is called, is well developed in most mammals and serves to protect the eye. In man its representative is the insignificant fold already described, which, devoid of muscles and other appurtenances, is in its present condition a purely useless part, a vestigial organ. It is a noteworthy fact that, though the nictitating membrane is a functional organ in many mammals, it is reduced to a semilunar fold not only in man but in the monkeys and the apes.
Vestigial organs in man are found not only on the exterior of the body, as shown by the eye and the ear, but in its interior. In human beings the small intestine is not continuous with the large one end to end, but the small intestine enters the side of the large intestine, the natural termination of which is a pocket that projects backward and is known as the caecum. This pocket carries on its surface a small worm-like attachment, the so-called vermiform appendix, whose cavity opens directly into that of the caecum. In a way the vermiform appendix marks the real ending of the large intestine (Fig. 7). As is well known, the appendix readily becomes a center of intestinal disturbance and is the seat of the disease known as appendicitis, the usual surgical cure for which is to remove the appendix. Thousands of human beings have had the appendix removed and continue to live without experiencing any inconvenience. In fact, their position in life after the removal of this troublesome organ is so much more secure than it was before that
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