THE PHYLOGENY OF THE HEXACTINELLIDA. 



After a detailed investigation of a group of animals, it is incumbent on every 

 naturalist who accepts the evolution theory to attempt the appreciation of his results 

 in theii- relation to the phylogeny of the group. 



In attempting to draw conclusions from the results of my investigation of living 

 forms, I am indeed conscious that such deductions as to pedigree cannot claim to have 

 any absolute certainty, and that the less since, in spite of the splendid work of Zittel, 

 the anatomical and embryological facts can be but slightly supplemented by the results of 

 palseontological research. Certain skeletal portions of fossil Hexactinellida, and especially 

 the connected framework, have indeed sometimes been found wonderfully preserved, and, 

 after separation from adjacent material by careful maceration in acids, have even 

 admitted of very intimate study. But the siliceous spicules occurring loosely in the 

 parenchyma, and specially important for diagnosis, are as a rule not preserved at all. 

 Of the soft parts there is furthermore no trace ; and, finally, the fossil remains that have 

 been found represent only some small divisions of the entire class. I must therefore 

 restrict myself simply to collating the conclusions drawn from the living foi'ms wdth 

 the few results of pal?eontological research, to show at least to what extent they 

 harmonise. 



In the discussion of the phylogenetic relations of living Hexactinellids to one 

 another and to known fossil forms, I shall have repeatedly to refer to the papers which 

 I have in the past year laid before the Berlin Academy.^ There can be no doubt, 

 to any evolutionist, that the close resemblance, both in general structure and 

 details of organisation, which is expressed in the grouping of difierent forms in 

 the systematic unities of species or even genera must be based upon, and explained 

 in terms of real relationship. But the greater the extent of the systematic cate- 

 gories, the greater usually are the gaps which are seen to exist in the living fauna, 

 and the more difficult does it become to determine the actual relationships. The con- 

 ventional method of representing the various forms in a continuous series is felt to be 

 much more insufficient when dealing with the larger divisions than when expressing the 

 relations of the usually simpler varieties of a species or of the members of a genus. 



' Abhandl. d. Kbnvjl. Preuss. Akad., 1886. 



(ZOOL. CIIALL. EXP. — PART LIII. — 1887.) Ggg 62 



