CLASSIFICATION. II 



are often wide dift'erences of opinion as to the number of species which 

 a genus contains. In a handful of small shells the "spHtters" may 

 recognise 20 species where the " slumpers " see only 3. Thus Hceckel 

 says of calcareous sponges that, as the naturaHst likes to look at the 

 problem, there are 3 species, or 21, or 289, or 591 ! 



But while no rigid definition can be given of a species, seeing that 



Fig. I. — Diagrammatic expression of classification in a 

 genealogical tree. B indicates possible position of Balano- 

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



the conception is one of practical convenience and purely relative, there 

 are certain common-sense considerations to be borne in mind — 



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



