CLASSIFICATIONS IN NATURAL HISTORY. 205 



Nicholson separates the Sponges from Coelenterata under 

 the branch name Porifera; inchides the Vermes and the Ar- 

 thropoda of Claus in one branch, the Annulosa, making of 

 them three sub-branches : I. Solecida, II. Anarthropoda (these 

 two sub-branches together constitute the branch Vermes of 

 Chius), and III. Arthropoda, which includes the same classes 

 as assigned to that division by Claus. 



The Classes of Importance in Paleontology and their Knowm 

 Range in Geological Time — Those classes which are of impor- 

 tance to the student of the history of organisms are the fol- 

 lowing: the names are used uniformly so far as to include the 

 same organisms, but their theoretical relations to each other 

 are not stated alike by different authors. (See next page.) 



Species and Genera of Chief Use in Tracing the History of Or- 

 ganisms. — When we come to the actual study of the historical 

 relations of organisms it is specific and generic characters with 

 which we chiefly deal, and the grouping of them into families, 

 orders, classes, and branches is the result of the study rather 

 than a matter of direct observation. 



We agree with Zittel * that the systems of classification in 

 biology are only the expression of our actual knowledge of 

 the reciprocal relations of the organisms: they depend di- 

 rectly upon the present state of our knowledge, and are sub- 

 ject therefore to more or less profound modifications. 



The higher categories are built up of generalizations de- 

 rived from comparison of the detailed structure of the indi- 

 viduals. All our systematic categories are artificial abstrac- 

 tions which rest upon the greater or less resemblance of 

 form in the individuals. The historical relations between the 

 characters marking these larger ca' egories are not matters of 

 observation, but only of speculation. The history is to be 

 observed in series of successive species, and the study of 

 classifications becomes of importance in restricting our at- 

 tention to the field within which all the evidence to be had 

 must be found. The actual evidence of the history, which 

 the paleontologist may see and examine, is presented in the 

 specific and varietal characters of the fossil remains preserved 

 in the rocks. 



* See " Handbuch der Palaeontologie," vol. i. p. 39, etc. 



