" Nature exhibits to us individuals succeeding each other, 

 but the species among them have only a relative sta- 

 bility, and are only temporarily invariable." LAMARCK. 



" We can only predicate and define species at all from 

 the mere circumstance of missing links. Our species 

 are twigs of a tree separated from the parent stem. 

 We name and arrange them arbitrarily, in default of a 

 means of reconstructing the whole tree, in accordance 

 with nature's ramifications." ELLIOTT COUES. 



iv 



