REPORT ON THE RADIOLARIA. C i 



CHAPTER VI. PHYLOGENY OR GENEALOGICAL DEVELOPMENT. 



( 153-200.) 



153. Sources of Phylogenetic Knowledge. For the purpose of constructing a hypo- 

 thetical genealogical tree of the Racliolaria, as of all other organisms, three sources of 

 information are open to us, viz., palaeontology, comparative ontogeny, and comparative 

 anatomy. In the present case, however, these three sources are of very different value ; 

 the first two are at present only very inadequately known and have only been partially 

 investigated, hence they can only be utilised to a very slight extent. The comparative 

 anatomy of the Racliolaria, on the other hand, is so completely known, and affords such 

 certain glimpses into the morphological relations of the related groups, that by its aid we 

 are in a position at all events to lay down the general features of their phylogeny with 

 some probability, and to lay the foundation of a natural system. 



154. Natural and Artificial Systems. Although in the classification of the Radiolaria, 

 as in the case of all other organisms, the natural system must be regarded as the goal of 

 systematic classification, our phylogenetic knowledge of the Radiolaria is too fragmentary 

 and inadequate to admit of the systematic arrangement here adopted being regarded 

 as a thoroughly consistent natural system, that is, as representing the true genealogical 

 tree of the class. Owing, however, to the extraordinary variety of form of the Radiolaria, 

 and the complicated relationships of the larger and smaller groups, a synoptical grouping 

 of the different categories and the erection of a complete, even if to some extent artificial, 

 system, becomes a logical necessity. Under these circumstances, and regard being had to 

 both these conditions, the following systematic treatment of the Radiolaria will appear as 

 a compromise between the natural and artificial systems, like all other zoological and 

 botanical classificatory attempts. On the one hand, the attempt is made to arrange the 

 larger and smaller groups as nearly as possible according to their phylogenetic relation- 

 ships, whilst, on the other hand, the practice of circumscribing each by a definition as 

 dear and logical as possible has been carried out. Since these two efforts naturally often 

 come into contact, the insufficiency of many parts of the arrangement is obvious, hence 

 its hypothetical and provisional character is emphatically stated. 



155. Systematic Categories. The categories or different orders of divisions have in 

 the Radiolaria, as in all other organisms, no absolute significance, but only a relative 

 value. In itself it is quite unimportant whether the whole group be regarded, as at first, 

 as a family (Ehrenberg, 1847), or as an order (J. Miiller, 1858), or as a clans (Haeckel, 



