DUAL CLASSIFICATION 15 



under no corresponding obligation, and it is 

 partly through losing sight of this circumstance 

 of the differential purport of morphology and 

 physiology, partly also through lack of know- 

 ledge of palaeozoic physiology, that much con- 

 fusion has arisen. 



Returning now to the consideration of the 

 dual classification of the animal kingdom : — the 

 current system, which has received so much 

 additional stability by the recent progress of 

 protozoology, recognises the two great sub- 

 divisions of the protozoa and the metazoa. It 

 is sometimes considered an advantage to unite 

 all unicellular organisms under the Haeckelian 

 term protista from which are derived the parallel 

 stems of the metazoa and the metaphyta (multi- 

 cellular plants). In the vegetable kingdom the 

 transition from unicellular to multicellular forms 

 is graduated even amongst existing natural 

 orders ; and the algae include both kinds. 



The protozoa are commonly treated as a 

 phylum of the animal kingdom equivalent in 

 classification to the other phyla, but this pro- 

 cedure is not in strict accordance with facts. 

 The protozoa occupy a position which is unique, 

 and they are rightly contrasted with the poly- 

 phyletic metazoa. There is no such abstraction 

 as a protozoan type in the same sense as there 

 is a molluscan type or a vertebrate type, 

 because the combination of characters which is 



