ADAPTATIONS 147 
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. 
82. 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 other 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 asan 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. 
Fie. 87.—Young stages of the mosquito. 
a, larva (wriggler) ; b, pupa. 
