200 ESSAY ON CLASSIFICATION 



stands out from its yolk. These simple facts, known since Sars and 

 Bischoff published their first observations, twenty years ago, is in it- 

 self sufficient to show that the whole principle of classification of 

 Vogt is radically wrong. 



Respecting the assertion that neither Infusoria nor Rhizopoda pro- 

 duce any eggs, I shall have more to say presently. As to the arrange- 

 ment of the leading groups, Vertebrata, Articulata, Cephalopoda, 

 MoUusca, Vermes, Radiata, and Protozoa in Vogt's system, it must 

 be apparent to every zoologist conversant with the natural affinities 

 of animals that a classification which interposes the whole series of 

 Mollusks between the types of Articulata and Worms cannot be 

 correct. A classification based like this, solely upon the changes 

 which the yolk undergoes, is not likely to be the natural expression 

 of the manifold relations existing between all animals. Indeed, no 

 system can be true to nature which is based upon the consideration 

 of a single part or a single organ. 



After these general remarks, I have only to show more in detail 

 why I believe that there are only four great fundamental groups in 

 the animal kingdom, neither more nor less. 



With reference to Protozoa, first, it must be acknowledged that, 

 notwithstanding the extensive investigation of modern writers upon 

 Infusoria and Rhizopoda, the true nature of these beings is still very 

 little known. The Rhizopoda have been wandering from one end 

 of the series of Invertebrata to the other, without finding a place 

 generally acknowledged as expressing their true affinities. The at- 

 tempt to separate them from all the classes with which they have 

 been so long associated and to place them with the Infusoria in one 

 distinct branch appears to me as mistaken as any of the former ar- 

 rangements, for I do not even consider that their animal nature is 

 yet proved beyond a doubt, though I have myself once suggested 

 the possibility of a definite relation between them and the lowest 

 Gasteropods.^ Since it has been satisfactorily ascertained that the 

 Corallines are genuine Algae, which contain more or less lime in 

 their structure, and since there is hardly any group among the 

 lower animals and lower plants which does not contain simple 

 locomotive individuals as well as compound communities, either 

 free or adhering to the soil, I do not see that the facts known at 



* Compare Chap. I, Sect, xviii. 



