CLASSIFICATION. 15 



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 

 not be markedly fluctuating. The specific characters should exhibit 

 a certain degree of constancy from one generation to another. 



2. Sometimes a minute chararti-r, such as the shape of a tooth or the 



FIG. 18. Diagrammatic expression of classification in a 

 genealogical tree. B indicates possible position of Balano- 

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



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 

 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 



