THE THEORY OF NATURAL SELECTION. 303 



(4) This view is further borne out by the common 

 existence in adult animals of what are known as 'rudi- 

 mentary organs,' or, in other words, imperfectly developed 

 organs which are of no use to their possessor. Thus, 

 ordinary Snakes do not possess either the fore or hind 

 limbs ; but the Boas and Pythons possess rudimentary 

 hind-limbs in the form of a pair of horny spurs. Again, 

 the Whalebone Whales have no teeth; but they exist 

 nevertheless in the young animal, though they remain 

 buried in the jaw and never cut the gum. The same is 

 true of the upper front teeth in Ruminant animals, which 

 also do not cut the gum, and are therefore of no use to 

 the animal. Another instance may be taken from the 

 whales, which show no signs of hind-limbs externally, but 

 which for all that often have the rudiments of these 

 limbs hidden internally. The only satisfactory explana- 

 tion of the general nature of rudimentary structures which 

 it seems possible to give, is that they are structures which 

 existed in a fully-developed condition in the remote 

 ancestors of an animal, but which have gradually 

 dwindled down in size and have lost their function 

 through long-continued disuse. Sometimes rudimentary 

 organs may be 'nascent' structures that is, structures 

 which in course of time may become functionally 

 useful to the animal; or sometimes they may merely 

 represent the atrophied condition of structures which 

 the embryo possessed ; but this does not affect the 

 above general explanation. Accepting this view, we 

 should judge that the whalebone whales were descended 

 from some type of Mammal which possessed teeth in 

 its jaws, and which was at the same time provided 



