A DEFINITION OF EVOLUTION 



an extinct flying reptile, had a wing which was formed by a fold of skin 

 stretched between the bod\% the posterior surface of the arm, and an 

 immensely elongated fifth digit, the other four digits remaining free. In 

 birds, the feathers which form the planing surface are inserted on all of 

 the three major segments of the appendage. The first digit is somewhat 

 independent of the others and can be moved separately by some birds. It 

 bears feathers. The second and third digits are fused together to form the 

 major skeletal basis of the distal part of the wing. The fourth digit is 

 accessory to these, and the fifth is missing altogether. In the bats, a fold 

 of skin again extends from the body to the arm to form the planing 

 surface. But this time only the first digit is free and of typical size, while 

 the other four are elongated to form the major supports of the wing. 

 Therefore, although the wings of all of these flying vertebrates can be 

 regarded as homologous when considered simply as vertebrate append- 

 ages, they are only analogous when considered as flight organs. Different 

 materials have been used for the adaptations to flight. 



Vestigial Organs. Another very persuasive aspect of comparative anat- 

 omy concerns vestigial or rudimentary organs. These are dwarfed and 

 generally useless organs which are found in many plants and animals, 

 relatives of which may have the same organ in a fully developed and 

 functional condition. Perhaps the most widely known example is the ver- 

 miform appendix of man (Figure 18). This small structure, without any 

 known function in man, is notorious as a seat of disease. In other Primates, 

 however, this organ is considerably larger. And in mammals which eat 

 a coarse diet, involving considerable amounts of cellulose, the appendix 

 and cecum form a larai;e sac in which mixtures of food and enzvmes can 

 react for long periods of time. The appendix of man is easily understand- 

 able as a degenerating legacy from ancestors with a much coarser diet; 

 but it is inexplicable why a useless and disease-ridden structure should 

 have been created especially to plague him. 



Weidersheim has listed nearly 100 such vestigial characters in man, and 

 a few of these may be mentioned. In the inner corner of the eve of all 

 vertebrates there is a transparent membranous fold, the nictitating mem- 



FiGURE 18. The Appendix in the Ohang-utan and in Man. il, ileum; co, colon; C, 

 caecum; W, window cut in caecum; +, appendix. (From Romanes, "Darwin and After 

 Darwin," Open Court Publishing Co., La Salle, HI., 1902.) 



42 



