312 



ANIMAL STUDIES 



to the influence of the struggle for existence. It is thus 

 the outside of an animal that tells where its ancestors 

 have lived. The inside, suffering little change, whatever 

 the surroundings, tells the real nature of the animal. 



261. Vestigial organs. In general, all the peculiarities of 

 animal structure find their explanation in some need of 

 adaptation. When this need ceases, the structure itself 

 tends to disappear or else to serve some oilier need. In 

 the bodies of most animals there are certain incomplete 

 or rudimentary organs 

 or structures which 

 serve no distinct use- 

 ful purpose. They are 

 structures which, in the 

 ancestors of the ani- 

 mals now possessing 

 them, were fully devel- 

 oped functional organs, 

 but which, because of a 

 change in habits or con- 

 ditions of living, are of 

 no further need, and 

 are gradually dying out. 

 Such organs are called 

 vestigial organs. Ex- 

 amples are the disused 

 ear muscles of man, the 

 vermiform appendix in 

 man, which is the reduced and now useless anterior end 

 of the large intestine. In the lower animals, the thumb or 

 degenerate first finger of the bird with its two or three little 

 quills serves as an example. So also the reduced and elevated 

 hind toe of certain birds, the splint bones or rudimentary 

 side toes of the horse, the rudimentary eyes of blind fishes, 

 the minute barbel or beard of the horned dace or chub, and 

 the rudimentary teeth of the right whales and sword-fish. 



FIG. 187. Young stages of the mosquito. 

 a, larva (wriggler) ; b, pupa. 



