THE THEORY OF CHANGE OF FUNCTION 61 



the mandibles, in most Arthropods completely 

 and permanently altered, both structurally and 

 functionally, the hinder ones being altered to less 

 extent. 



Of the two divisions of the limb the inner or 

 endopodite is the nearer to the mouth, and conse- 

 quently most likely to be useful for purposes of 

 feeding. On the other hand, for swimming, the 

 exopodite is of equal, if not greater, importance ; for 

 walking the endopodite is of most use as being more 

 directly beneath the body. Another good example 

 of change of function is found in the hyomandibular 

 cleft. This, like certain other clefts, has become 

 saved from destruction by becoming modified into 

 an accessory organ of hearing. 



The olfactory organ furnishes another illustration 

 of the theory, and with regard to this let us first 

 consider what appear to be the difficulties of the 

 case. All Vertebrates possess olfactory organs with 

 the solitary exception of Amphioxus, that curiously 

 exceptional Vertebrate whose anatomy seems to be 

 made up of contradictions. These olfactory organs 

 have in all cases the same essential structure. 

 Certain differences occur, but these are so slight as 

 to leave no doubt whatever that the vertebrate 

 olfactory organs are, wherever they occur, the 

 same organs. As soon, however, as we get beyond 

 Vertebrates we meet with a difficulty. There is no 

 doubt that Vertebrates came into existence later 

 than Invertebrates, therefore Vertebrates must either 

 have inherited their olfactory organs from their 

 invertebrate ancestors, or must have acquired them 



