468 



ELEMENTARY BIOLOGY 



suggestions of structures found in the life history of birds and 

 of fishes. Now the only explanation of these facts that appears 

 at all reasonable is that there is a common (or, similar) develop- 

 ment just to the extent that organisms are related through 

 descent from common ancestors. 



491. Vestigial structures. Another line of evidence is found 

 in the presence, among plants and animals, of certain organs that 



FIG. 250. Vestigial structures 



The vermiform appendix, a, in some mammals is reduced to an insignificant trifle, as 

 in man, / ; whereas in other mammals, as in some of the rat family, 2, it is capable of 

 holding a considerable amount of food in the process of digestion. The horse walks 

 on his third toe, j, the others being entirely absent or represented in part by the 

 reduced " splints," b 



are quite useless from the point of view of adaptation, but which 

 are nevertheless persistent through whole groups. For example, 

 the whale develops legs that are never used, and the same is 

 true of certain snakes. The skeleton of many a bird shows dis- 

 tinct signs of fingers, or claws, among the wing bones. 1 Other 

 examples can be readily understood if we suppose that all plants 

 and all animals are related through having had common ances- 

 tors ; but they cannot be understood on any other supposition. 



1 The vermiform appendix (see /, Fig. 28) in man is the lingering reminder 

 of an organ that developed and functioned in other backboned animals, but 

 that has no practical meaning in the life of man to-day except to make 

 trouble sometimes. 



