44 INTRODUCTION TO EVOLUTION 



How are we to explain the presence of useless structures such as those 

 described above? Are we to suppose that creatures were "deliberately" 

 made with structures which would never be of use to them? Or does it 

 seem more reasonable to conclude that the kiwi, for example, inherited its 

 wings from an ancestor which was a flying bird and hence had use for 

 wings? 



As already mentioned, occasional biologists doubt that structures usu- 

 ally classed as rudimentary are in fact without function. It has been main- 

 tained, for example, that the small bones we have spoken of as rudimen- 

 tary hind limbs in whales are not such at all, but are bones having the 

 function of stiffening the walls of the anus, the posterior opening of the 

 digestive tract. Most students of anatomy are not in accord with this view. 

 Occasional mistakes may be made in labeling small organs as rudimentary, 

 but it seems entirely unlikely that the percentage of error is high. To 

 most biologists, therefore, the presence of small organs that seem to have 

 no function in themselves but correspond to functional organs possessed 

 by other animals indicates inheritance from common ancestry. Descend- 

 ants having use for the organ in question retained it as a functional or- 

 gan; in descendants having no use for it the organ became reduced in size. 

 The culmination of this trend would be complete loss of the organ. Ap- 

 parently some extinct flightless birds, such as the giant moa of Madagas- 

 car, attained this extreme. Possession of rudimentary organs may be re- 

 garded as a way station on the road to elimination of those organs. 



References and Suggested Readings 



Darwin, C. The Origin of Species by Means of Natural Selection. 1859. Modern 



Library series, Random House, New York; or Mentor Book MT294, New 



American Library, New York. (Note particularly Chap. 14.) 

 Dewar, D. Difficulties of the Evohition Theory. London: E. Arnold & Co., 



1931. 

 Guyer, M. F. Animal Biology, 4th ed. New York: Harper & Brothers, 1948. 

 Lull, R. S. Organic Evolution, rev. ed. New York: The Macmillan Company, 



1947. 

 Romer, A. S. The Vertebrate Body. 2nd ed. Philadelphia: W. B. Saunders Co., 



1955. 

 Snodgrass, R. E. Principles of Insect Morphology. New York: McGraw-Hill 



Book Company, Inc., 1935. 

 Walter, H. E., and L. P. Sayles. Biology of the Vertebrates, 3rd ed. New York: 



The Macmillan Company, 1949. 

 Zangerl, R. "The methods of comparative anatomy and its contribution to 



the study of evolution," f'vo/M/Zon, 2 (1948), 351-374. 



