196 LIFE ON THE EARTH. 



be held to mean independent permanency of genus, 

 and dependent mutability of species we remark in 

 this passage the recognition of general 'primary 

 types,' the 'struggle for existence' and the final 

 'limitation of species,' which have often been ap- 

 pealed to in later speculations. 



DISTINCTION OF SPECIES. 



Ever since the days of Linnaeus the tendency of 

 naturalists has been toward a nicer discrimination 

 of species and a more thorough appreciation of the 

 affinities which constitute natural assemblages. The 

 former analytical process has been pushed very far 

 in Botany, Zoology and Palaeontology, so that what 

 Linnaeus and his immediate followers regarded as 

 mere varieties have since been freely admitted to 

 rank as 'good' species. This decision in regard to 

 the majority of plants and animals has been quite 

 independent of and without any reference to proof 

 of the descent of the objects from a common 

 parentage which is the original ground of specific 

 identity, or from different parents which should con- 

 stitute specific difference. In the case of a very 

 large majority of the living invertebrata, no proof 

 of this sort has been looked for; a definite pecu- 

 liarity of structure, however slight, if often repeated, 



