CLASSIFICATION. 15 



I. No naturalist now believes, as Linnaeus did, in the fixity of species ; 

 we believe, on the contrary, that one form has given rise to another. 

 At the same time, the common characteristics on the strength of which 

 we deem it warrantable to give a name to a group of individuals, must 



FlG. 1 8. Diagrammatic expression of classification in a 

 genealogical tree. B indicates possible position of Balano- 

 glossus, D of Dipnoi, S of Sphenodon or Hatteria. 



not be markedly fluctuating. The specific characters should exhibit 

 a certain degree of constancy from one generation to another. 



2. Sometimes a minute character, such as the shape of a tooth or the 

 marking of a scale, is so constantly characteristic of a group of indi- 

 viduals that it may be safely used as the index of more important 



