CLASSIFICA TION. 



IS 



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

 not be markedly fluctuating. The specific character 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- 



Fig. 18. — Diagrammatic expression of classification in a 

 genealogical tree. B indicates possible position of Balano- 

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



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

 characters. On the other hand, the distinction between one species and 

 another should always be greater than any difference between the members 

 of a family (using the word family here to mean the progeny of a pair). 

 For no one would divide mankind into species according to the colour 

 of eyes or hair, as this would lead to the absurd conclusion that two 



