42 INTRODUCTION TO EVOLUTION 



diet consists of both plant and animal material, but in the preparation of 

 plant material for human consumption we eliminate most of the cellulose 

 (in the "woody" portions). We do not use our caecum and appendix as a 

 container for food undergoing bacterial action. Then why do we have 

 them? The most reasonable explanation seems to be that we inherited 

 them from some remote ancestor having a diet which necessitated such 

 adjuncts to the digestive system. When the descendants of this ancestor 

 eventually changed their food habits the caecum and appendix, no longer 

 useful, decreased in size until they became mere remnants of the func- 

 tional organs they once had been. 



It is difficult to explain the presence of useless vestiges upon a basis of 

 special creation without imputing to the Creator some lack of skill in plan- 

 ning or construction. Accordingly, opponents of the idea of evolution 



nicti tenting membrane 



semilunar fold 



mi0^ 



OWL 



FIG. 3.13. Nictitating membrane (third eyelid) of owl and horse, and vestigial semilunar 

 fold of man. (Mainly after Romanes, Darwin and After Darwin, 3rd. ed.. Open Court 

 Publishing Company, 1901.) 



commonly maintain that organs like the appendix are not useless at all, 

 that they have functions which we have never been able to discover. 

 Clearly, "the burden of proof lies with the affirmative" in the matter of 

 proving the usefulness of vestiges for which no functions have ever been 

 discovered. Many readers can testify from personal experience that if the 

 appendix has a function at all it is so unimportant that the advantages of 

 having the organ removed far outweigh the disadvantages. 



Other vestiges are found in the human body. For example, in the inner 

 angle of each of our eyes there is a little fold of flesh called the semilunar 

 fold (plica semilunaris) (Fig. 3.13). This corresponds to a structure 

 which in many lower animals is a movable third eyelid, the nictitating 

 membrane, lying under the other eyelids and sweeping across the eye 

 from the inner angle outward. In many animals, as for example owls, the 

 nictitating membrane is transparent, affording a means of cleaning and 

 lubricating the surface of the eyeball without obstructing vision in the 

 process, even for the fraction of a second necessary to wink the other two 



